A manga adaptation was published in three issues of Comic Yuri Hime. The manga shared the same characters and setting as the anime, but presented a different storyline. A second manga was serialised in Megami Magazine, with a radically different setting and a comic rather than serious and dramatic tone. There is also a two-volume light novel adaptation, which unlike the manga has a storyline close to that of the anime.
In May 2007, Simoun has been licensed for release in North America by Media Blasters.
At the start of the series, the theocratic nation of has a monopoly on technology left over from the fallen ancient civilization, which it uses to power aircraft and trains. The industrialized nation of , located on an archipelago across the ocean from Simulacrum, wishes to steal the helical motor technology in order to further their own technological advancement and imperialist ambitions. Argentum possesses technology similar to that of industrialized nations in the 1920s or 1930s, including mass-produced lighter-than-air rigid zeppelins and heavier-than-air propeller-driven biplane fighter aircraft, and as the series begins, it is preparing to attack Simulacrum with a fleet of fighter-carrying airships. Other neighbouring countries, such as the , also have hostile relations with Simulacrum due to religious differences or faltering economies.
Simulacrum is defended by advanced airships known as "Simoun." A Simoun is a two-seater aircraft propelled by two helical motors, based on technology left over from the fallen ancient civilization. The exact operation of the helical motor is not well understood by Simulacran technicians, but one of the two rotors apparently controls the Simoun's position in space, and the other affects the flow of time. According to their religion, two priestesses descended from heaven and explained to the people how to operate the Simouns, after they were found in excavations or had just been abandoned. These same priestesses are credited for teaching the people anew about Tempus Spatium and renewing the faith and religion. A Simoun requires two crew members to fly. The two crew members have separate cockpits, each with its own independent flight controls and instrumentation. The crew member in the forward cockpit, known as the auriga, actually flies the Simoun, while the crew member in the aft cockpit, known as the sagitta, acts as a navigator and controls the Simoun's weapons and other accessories (however, at times in the series it seems the sagitta affects the flying of the craft as well. In episode 5, the Simoun flown by Rimone and Aer flies off-course during the Bell Ri Mājon. The fault is attributed to Rimone, even though she is the sagitta.). Simulacrum also possesses one- and two-seater training aircraft similar to the Simoun, known as , which are based on a less-advanced design with one somewhat simpler helical motor.
Simulacrum's religion is centered on the worship of a spiritual force known as . Tempus Spatium is depicted in Simulacran iconography by winged pillars or goddesses. One of the most important centers of worship of Tempus Spatium is "the Spring", an underground grotto containing a ruined temple to the Tempus Spatium. Simulacran girls make a pilgrimage to the Spring when they turn seventeen; entering the Spring in a particular ritual context with a firm intention in mind causes the pilgrim to make a final selection of their permanent sex. Through a mechanism which is not fully understood (the Simulacrans believe it to be the power of the Tempus Spatium), the individual's DNA is rewritten to confirm or reverse their gender.
The Simoun are crewed by priestesses called (plural; singular Sibylla). Each Simoun Sibylla is a girl who has not chosen a permanent sex yet; doing so permanently disqualifies one from serving as a Sibylla. Some Sibyllae have delayed their choice of gender past the age of 17; due to the war, they are granted special dispensation to do this. The Sibyllae understand their use of the Simoun to be sacred to Tempus Spatium; a form of elaborate prayer. The main weapon of the Simoun is a large glowing green gem, (the "Simoun Gem) positioned between the cockpits, which produces an effect known as when the Simoun is flown in the correct pattern. The Simoun Gem is activated before takeoff when the two Sibyllae kiss it after kissing each other.
The Sibyllae believe that inscribing enormous glyphs in the sky using the Simoun Gem invokes the power of Tempus Spatium to produce powerful magical effects. Different Ri Mājon glyphs can be combined to produce a nearly infinite variety of effects, but most of the lore surrounding the language of the Ri Mājon has been lost over time. As the series begins, the Simoun Sibyllae are working to try to rediscover this ancient knowledge. Different Ri Mājons have different effects and functions. There are combat Ri Mājons, diplomatic Ri Mājons, funeral Ri Mājons, and so forth. Ri Mājons differ widely in the number of craft required to complete them, and in the time and skill needed to successfully inscribe them.
Sibyllae are organized into "choirs" of twelve priestesses. A choir at full strength operates six Simoun, which is sufficient to allow them to inscribe the most complex known Ri Mājon patterns in the sky. Simoun choirs take the title (Greek for "choir"), and a name derived from Latin or a language very similar to it. "Chor" is a homonym for "corps", and the resulting double religious and military meaning may or may not be intentional. A Simoun choir is led by a senior Sibylla known as the . The most senior Sibylla in the entire Simoun fleet is known as the . A group of one or more choirs is commanded by a , who is an adult religious official who has chosen a permanent sex, and therefore is unable to operate a Simoun himself.
The series focuses on one particular Simoun choir, , which has a reputation as an elite unit. Chor Tempest is one of three choirs based on a large airship, the , which functions as a flying aircraft carrier.
As the series opens, Argentum sends a large fleet of airships to attack Simulacrum, in order to steal the helical motor technology found in the Simoun. Fifteen or more carrier airships participate in the attack, yielding a total force of hundreds of "dragonfly" fighters. The invasion fleet is intercepted by two Simoun choirs on routine patrol: and Chor Tempest. Simulacran doctrine holds that any potential enemy will flee in terror at the first sight of a Simoun, yet the Argentine air fleet manages to completely wipe out Chor Caput. Chor Tempest is able to destroy large number of enemy fighters with its own Ri Mājon power before being lured into an encounter with the entire Argentine fleet. They attempt to destroy the whole fleet with a powerful maneuver known as the , but the Argentine fighters are so numerous that they wind up erasing the Ri Mājon trails. One of the choir's Simoun is destroyed by enemy gunfire, and the remainder are left unable to execute a Ri Mājon effect sufficient to repel the enemy attack. In desperation, Neviril and her partner Amuria attempt an extremely powerful but extremely dangerous maneuver, the , in a last-ditch attempt to salvage the situation.
Neviril and Amuria appear to be on the verge of successfully completing the Emerald Ri Mājon when disaster strikes. Their Simoun has a head-on encounter with an Argentine dragonfly fighter. Neviril sees the enemy pilot inside his cockpit and meets his gaze, which forces her to see the attacking enemy forces as people rather than machines. She loses the will to kill them, and her resulting moment of hesitation causes the Ri Mājon to fail. The failure of the Emerald Ri Mājon causes an explosion similar to the detonation of a nuclear weapon. The remaining Simouns of Chor Tempest escape the blast, but the entire attacking fleet is annihilated, and Neviril's Simoun is crippled. The upper fuselage is a twisted wreck, and the sagitta cockpit is missing entirely and Amuria with it.
Three Sibyllae from Chor Tempest were killed in the initial battle. After the battle, three more announce their intent to leave the ranks of the Sibyllae and go to the Spring to choose a permanent sex. The remaining Sibyllae are all too demoralized to act as an effective fighting force, especially Neviril, who cannot even bring herself to leave her stateroom on the Arcus Prima. The only bright spot for Chor Tempest at this time is the arrival of two promising new recruits: Sibylla Aer, an excellent pilot with unshakeable morale, and Sibylla Rimone, a child prodigy who was recently made the youngest Sibylla ever. Aer immediately decides to try to form a pair with Neviril, but Neviril is still grieving for Amuria. Neviril follows one of the three departing Sibyllae to the Spring out of friendship, and nearly enters it herself.
Over the following weeks, Argentum continues its harassment of Simulacrum by sending fighter probes, conducting a powerful but inaccurate long-range bombardment intended to bring down the Arcus Prima, and sending agents into Simulacran territory to attempt to ambush and steal a Simoun. A second front to the war opens when Plumbum launches a mechanized invasion of Simulacrum over their shared mountainous border. Chor Tempest slowly tries to regain its confidence during this period, adding two new recruits: the flirty Sibylla Morinas and the mysterious and politically well-connected Sibylla Dominūra. Dominūra, the eldest of all the Simoun Sibyllae, is brought in to try to revive Chor Tempest, and she takes an immediate interest in Rimone. Aer persists in her attempts to get Neviril to pair with her, but Neviril remains mired in severe depression, and lets Chor Tempest drift as she wallows in her grief. Aer and Rimone, with Dominūra flying sagitta for Rimone, defeat the Plumbish armoured attack, leading the Plumbish government to sue for peace.
As the Arcus Prima heads for international waters to serve as the site of the peace conference between Plumbum and Simulacrum, Neviril's father Halconf interferes to try to jar his daughter out of her hikikomori phase. He orders two Sibyllae, Mamina and Yun, to join Chor Tempest to bring it to full strength, and on the strength of Mamina's service record, assigns her to be Neviril's new roommate and pair. Neviril, who is beginning to show signs of warming to Aer, rejects Mamina, and in reaction, Mamina attacks Aer. This shocking breach of conduct leads to the announcement that Chor Tempest will disband after the conclusion of the peace conference.
Relieved of its duties, Chor Tempest is assigned to play host to one of Plumbum's priestesses during the peace conference. The foreign priestess, Angulas, gets along well with the girls of the Simoun choir, but turns out to be a suicide bomber. She destroys two thirds of the Arcus Prima's Simoun wing and cripples the carrier itself. The bombing jars Neviril out of her funk. She agrees to pair with Aer, and leads Chor Tempest to save the Arcus Prima from certain destruction at the hands of a Plumbish battleship.
Now safe, Arcus Prima is brought into drydock at the Grand Temple. Neviril and Guragief travel to the chambers of the High Council of Simulacrum to try to defend Chor Tempest against Halconf's attempt to disband it. Neviril makes an impassioned speech before the council that offends many council members in its reference to the conflict as a "war", and in Neviril's refusal to blindly follow "the will of God" as Angulas did. Onasia, however, intervenes on Neviril's behalf, as does Sibylla Vura from the rival Chor Rubor. Vura and Neviril return from the Council chambers just as the remaining members of Chor Tempest are performing a funeral Ri Mājon in memory of the victims of the suicide bombing and right as Argentum launches a cruise missile attack against the Grand Temple. Chor Tempest easily destroys the missiles with the Iron Ri Mājon, and celebrates the news that the choir is not to be disbanded after all.
That evening, Chor Tempest learns that Sibylla Dominūra has found the choir a new ship to fly from while Arcus Prima is in drydock. Dominūra, it turns out, had secretly attended the hearing as well, and had met with a secret faction of officials opposed to Halconf's administration. Dominūra's faction was able to bring the old converted freight airship Messis out of retirement and refit her in the fashion of a World War II escort carrier. Conditions aboard the Messis are considerably more spartan than on the converted luxury liner Arcus Prima, much to the displeasure of the Sibyllae. Over the following weeks, Messis operates in a considerably more aggressive posture than Arcus Prima did, sparring with enemy patrols, participating in an airmobile assault with a unit of infantry, and intercepting an Argentine base ship.
Messis' active prosecution of the war is not matched by those in charge back at headquarters, however. Despite the discovery of a new class of heavy fighter based aboard the base ship and the possibility of an alliance between Plumbum and Argentum, Messis and Chor Tempest receive orders to passively monitor the base ship's activities rather than attacking. Dominūra takes matters into her own hands, ordering Aer and Neviril to sortie to intercept the base and conduct an armed reconnaissance of it. She also orders Wapōrif to dismantle one of the hitherto sacrosanct Simoun to learn the secret of how they work. Wapōrif is shocked at this violation of one of Simulacrum's strongest taboos, but eventually acceeds. His faith is shaken when he sees nothing obviously magical or divine inside the Simoun's helical motors; they appear to be mere machines to him. Dominūra, however, is able to see something inside that Wapōrif (who, being male, has already gone to the Spring) cannot -- something profoundly shocking. She has to be helped away, screaming, from the flight deck.
The enemy, meanwhile, is far from idle. A series of feints by enemy fighters manages to draw out four of Chor Tempest's six Simoun from the Messis in a futile attempt at a decisive engagement. Rimone manages to rouse Dominūra from her shock, but with only those two and Aer and Paraietta left to defend Messis, the enemy springs their trap. A large force of the new heavy fighters launches an alpha strike against the Messis during a thunderstorm. Only a quick decision by Wauf, its captain, to hide the ship, and the two remaining Simoun, on the ground in a forest behind a cliff from the enemy's approach vector, saves the ship from destruction. When the weather clears, the enemy reappears to resume their attack, and the two Simoun are compelled to intercept them to prevent them from discovering the Messis.
Although both Simouns manage to destroy a portion of the attacking force, the enemy has adopted new tactics that disperse their aircraft to make them less vulnerable to the Ri Mājon effect. The two Simouns destroy a portion of the enemy fighter wave, but are pursued so closely by the surviving fighters that they are unable to complete further Ri Mājons. Wauf orders the Messis to exit its position of concealment, thereby drawing the enemy fighter attack onto his own ship to buy the Sibyllae some time. In a desperate response, Rimone and Dominūra perform the Emerald Ri Mājon and succeed. Their Simoun vanishes without a trace in a flash of light as the rest of the choir -- including the four missing Simouns, who happen to appear just in time to see it -- look on in horror. Unlike in the first battle, however, the Emerald Ri Mājon has no effect on the attacking aircraft or the outcome of the battle, but the day is saved when Arcus Prima, now fully repaired and operational, conveniently appears to drive away the remaining attackers.
Back aboard the Arcus Prima, three government representatives (including Halconf) meet with the remaining members of the choir to discuss their next objective: to secure Simulacrum's holiest site, the Ruins, before the allied forces can steal Simoun-grade helical motors from it. They arrive to find three unidentified Simouns, of a different design to the Simoun that the Simulacran choirs use, exiting from the Ruins' main entrance. In the ensuing air battle, Chor Tempest manage to shoot down one of the stolen craft, but the other two escape unharmed.
During the battle, Aer and Neviril discover that Aer's music box, though closed and deactivated, plays its song around the entrance to the Ruins. Instead of joining the dogfight, they land to investigate... and discover the Spring inside, and Onasia. After the fight, the remaining members of the choir land, and discover another of the ancient Simoun inside the Ruins. Inside it, they find Angulas' dead body sitting in the auriga seat. The Simulacran religious authorities interpret this to mean that Tempus Spatium had forgiven Angulas, and they bury her with full honors. Mamina and Aer are excluded from the funeral due to their social class.
After the funeral, Aer and Neviril discuss what they saw at the Ruins. Aer reveals that her grandfather told her stories about a "door to another world" that the Simoun could open. She speculates that Angulas may have traveled to the ancient Simoun through that other world during the explosion. They compare this to Onasia's statements that she is "everywhere and nowhere" and that the Spring exists outside normal reality, and conclude that what's true for Angulas and Onasia may also be true for Dominūra and Rimone -- who therefore could be alive somewhere. Neviril swears Aer to secrecy about their discussion, and the two agree to keep it a secret.
Realizing that the Simouns seen at the Ruins were most likely piloted by Plumbish priestesses, the Simulacran Defence Minister orders Chor Tempest to locate and destroy the airborne supply base that Messis had discovered, using the Messis as their base. Aer and Yun are ordered to stay behind, with Mamina taking Aer's place as Neviril's pair. They ignore their orders and follow along in Chor Tempest's wake anyway. Neviril splits her patrol in half, taking Alty and Floe with her, and assigning a different sector to the other two Simouns.
When Neviril's flight locates the allied base ship, the two ancient Simouns that had escaped the dogfight at the ruins sortie to met them, along with a large number of Argentine heavy fighters. The two Simulacran Simouns destroy a number of opposing fighters, but the weight of numbers is too great, especially when combined with the presence of hostile Simouns. Completely defensive and unable to break out of evasive maneuvers long enough to execute a Ri Mājon, Neviril orders Alty and Floe to break off and summon reinforcements. She and Mamina barely evade a Ri Mājon attack by the enemy Simouns, but are promptly hit by gunfire from an Argentine fighter, and crash on the flight deck of the base ship. Neviril is incapacitated in the crash.
The two ancient Simouns land next to Mamina and Neviril's crippled craft, and the four Plumbish priestesses who were piloting them emerge, wielding pistols. They explain that they hold the Simoun Sibyllae in great reverence, and regret having to fight them. They try to help Neviril and Mamina escape. Instead, Mamina sacrifices herself to save the foreign priestesses, who would have been executed as traitors for helping them escape. The rest of Chor Tempest arrives with the Messis and scatters the allied troops on the base ship's flight deck, but it is too late -- Mamina had been shot through the heart by one of the priestesses to avoid the dishonor of being killed by the Argentine soldiers.
Roatreamon is disconsolate. She and Mamina had become extremely close in the weeks leading up to Mamina's final battle. To add insult to injury, the class-conscious Defence Ministry authorities promptly pack Mamiina's body off to her hometown rather than giving her the sort of elaborate funeral that Angulas received. And on the way, the Simile carrying her body is shot down by the two Plumbish Simoun.
The authorities give Chor Tempest little time to grieve. Aer, Neviril, and (at her request) Yun are delegated to accompany Onasia and several other religious officials to the Ruins, aboard the Messis. At the Ruins, the sibyllae are told the secret of the Emerald Ri Mājon: it allows the Simoun to travel in time. Chor Dextra was formed for that purpose, and used the Ruins as its training ground. In a last-ditch effort to save the rapidly-failing Simulacran war effort, the chief priestess of the Grand Temple orders Aer and Neviril to reactivate Chor Dextra and perform the Emerald Ri Mājon to prevent the war from occurring in the first place.
With the secret of the Emerald Ri Mājon revealed, we are finally shown what happened to Rimone and Dominūra. They landed in the far past, in the aftermath of a destructive war and a long period of lean economic times. Humanity had forgotten the secrets of the Simoun and the ways of their faith. The ancient Simoun found at the Ruins sit idle by the edge of the town where the two land, reduced to mere playthings for children. As the two are summoned to explain themselves to the townspeople, Dominūra explains to Rimone what she saw in the helical motors that upset her so much: she saw herself landing in this very town, explaining to the townspeople about the Emerald Ri Mājon, and thereby causing the entire war and all of the ensuing suffering. She asks Rimone for advice, and Limone explains that if there had not been Simouns, she never would have met Chor Tempest or Dominūra. This admission overcomes Dominūra's reservations, and history unfolds exactly as Dominūra and Rimone remember it.
The departure of Aer, Neviril, and Yun to the Ruins leaves Chor Tempest dangerously understrength, even when reinforced by Vura, who joins the choir to replace Mamina. Before the Messis can return, a black Simoun from the Arcus Niger crash lands on Arcus Prima's flight deck, with the news that the Niger has been destroyed by the alliance. Hard on its heels are not two but five Plumbish Simouns. The rump Chor Tempest gamely sorties to intercept them, only to see the Plumbish choir promptly disengage rather than destroy the sibyllae and the Arcus Prima. Arcus Prima returns to the Grand Temple in company with the Messis. Shortly after they arrive, the alliance launches an all-out attack on the Grand Temple itself. Chor Tempest sorties to intercept the assault. They manage to destroy several waves of incoming cruise missiles. Without the ability to do the Iron Ri Mājon, however, they are unable to intercept them all, and the surviving missiles detonate throughout the temple and surrounding city. Once again, the Plumbish Simouns refuse to engage in direct combat against the Sibyllae.
Despite Chor Tempest's partial success, the Simulacran authorities realize that the war is lost, and sue for peace. The allied peace terms include Plumbish and Argentine occupation troops aboard the Arcus Prima and the disbandment of Chor Tempest. Halconf is forced to resign his office as a result of the defeat. The Grand Temple's chief priestess pulls Neviril aside to urge her to take either Aer or Paraietta and perform the Emerald Ri Mājon to undo the war, and is promptly arrested by Argentine troops.
While the final peace agreement is being imposed, Yun takes a Simile and flies off to the Spring to talk to Onasia. Onasia confesses to having been a member of Chor Dextra, the last survivor, and to having committed some "sin" because she tried to avoid making her gender choice. Specifically, she has never chosen a sex, in an attempt to remain an "eternal maiden" -- just like the Simoun. She removes one of her gloves to reveal glowing bluish skin beneath, which sheds a trail of gold sparkles similar to a Simoun's Ri Mājon trail. Being the keeper of the Spring, Onasia explains, is her punishment, and she has not been able to touch or hold anyone since. Yun, after briefly flying back to the Arcus Prima to discuss the matter with Rodoreamon, takes pity on Onasia. She returns to the Spring to embrace Onasia and offers her forgiveness. This breaks the spell, and Onasia dissolves in a shower of golden sparks. Yun takes up Onasia's office as the new keeper of the Spring.
The remaining sibyllae of Chor Tempest are promptly packed off to the Spring at the insistence of the allied forces. Aer and Neviril ask for more time, and stay behind, during which time they make up from a personal squabble and form a deep pair bond. Their other seven choir-mates catch the next train to the Spring and are shocked to find Yun standing in Onasia's place. Hand in hand, the seven wade into the Spring to make their respective gender choices, supported emotionally by Yun.
When the seven ex-Sibyllae return to the Arcus Prima to say their goodbyes, the allied occupation forces are alarmed to learn that two Simulacran sibyllae remain who have not gone to the Spring. Aer and Neviril are promptly arrested and thrown in the ship's brig, to be taken to the Spring under close guard. Their former choirmates unsuccessfully try to break them out. Anubituf and Guragief have better luck. They conspire with the Plumbish priestesses who have relieved Chor Tempest of their duties, and manage to gather everyone by the Māju pool to say their farewells to Neviril and Aer as they go to the Spring. All at once, the Plumbish priestesses overpower the allied guards, and two more fly a Simoun up through the Māju pool to turn over to Aer and Neviril for their journey to "another world." The pair say their farewells and fly off into the sunset. The Plumbish authorities send a choir of Simouns to intercept Aer and Neviril, but instead of attacking, they perform the Ri Mājon of the Morning Calm in a friendly gesture of farewell. Aer and Neviril perform the Emerald Ri Mājon, and a peaceful white light appears at its center in place of the howling vortex from the other attempts. The two vanish in a flash of light.
Back in the past, a pair of new Simoun Sibyllae that Rimone has trained are finally ready to attempt the Emerald Ri Mājon. Dominūra is bedridden with some sort of illness, much to Rimone's dismay. The success of the Emerald Ri Mājon by Rimone's protegées reawakens a desire in Rimone to fly again. When another of the new Sibyllae pulls her aside to ask her to pair with her, Rimone turns her down. Dominūra is Rimone's pair, she explains, and she will have no other. At that moment, a Simulacran-style Simoun appears in the sky via the Emerald Ri Mājon and vanishes. "Aer!" exclaims Rimone. "Is that someone's name?" asks the Sibylla that Rimone turned down. "It's... the ultimate form of love," replies Rimone. "Yes, the ultimate love," says Dominūra, up from her bed and fully dressed. "To what sky shall we fly next?" she asks. Dominūra and Rimone embrace, and walk happily off to their Simoun to attempt the Emerald Ri Mājon again. As they walk off, the disappointed Sibylla notices sparkly gold flakes in Dominūra's wake. Yun calls out Dominūra's name as she peers into the Spring, years later.
Years pass. Plumbum and Argentum have a falling out, and a cold war develops between them. Paraietta manages an orphanage, funded by Roatreamon's family business. Alty and Kaim reconcile, and move back in with their parents. Guragief and Anubituf stay together. Morinas and Wapōrif marry and have several children. Wauf remains as the captain of the Messis, which reverts to being a freighter again in peacetime. Eliph and Morinas join the Messis' crew, and as they load cargo one day, Morinas briefly catches sight of Aer and Neviril's Simoun as it fades in and out of reality. Vuraf and Floef are drafted by opposite sides for the coming war, and Vuraf comes to visit Floef on his farm by the shore of the lake where the wreck of the Arcus Prima has settled. The two part with a wish that they never meet on the battlefield, and as the sun sets, Floef catches sight of an ancient V-tailed Simoun as it appears in the sky.
In the red glow of the sunset, a record player starts up in the deserted ballroom of the wrecked Arcus Prima. Neviril and Aer dance happily in each other's arms. As they dance, the ballroom is transformed back into its old self, until the pair vanish.
Friedman praised the Media Blasters release for having a mostly very good translation, for leaving in the honorifics (although no explanation was given for the honorifics on the disc). Friedman criticized the Media Blasters release for having strange names, which were insisted upon by the Japanese owners and for having no insert pictures.
Anime News Network praised the story for not having to rely on its yuri content and for being a creative mecha series that provides food for thought. Its tone is different from other teen pilot series, as it is "never grim". The characters are also noted as being more than "one-note cookie cutter portrayals". The use of penciled still shots remaining on the screen for some time was criticized. The series' resolution was perceived as partially satisfactory, as some elements were "bitter".