Dan's ship crashes into a lake of molten lead on Mercury, from which the crew are rescued by the planet's extremely strong inhabitants. They are then tied up, and taken away in a car made of crystal. Elsewhere on Mercury, this is watched on a televiewer by the Mekon.
The Mercurians throw their captives down a chute, apart from Digby and Willie the dog. The others are taken by other Mercurian transports to a city, where they can breathe inside without their space helmets, and the Mercurians with their natural air sacs deflated. There, they find Digby, who shows them a Treen helmet badge that the Mercurians gave him. Sondar suggests that it could be from Treens still loyal to the Mekon.
The Mekon sends troops to collect Dan and his crew. A guard gives Dan the Mekon's compliments, and asks Dan to accompany him to the Mekon. Though Dan's side win the ensuing fight, Digby and Urb are paralysed by gas in the process. The others try to seize a Treen ship, but it takes off with only Dan and Sondar on board. Peabody manages to escape back into the city, leaving Digby and Urb, but is captured by the Mercurians.
Dan and Sondar manage to seize the ship from her pilots and return to the city. There, they meet Peabody, who has learned some of the Mercurian language. The Mercurians cannot pronounce consonants, so their language is dependent on the musical notes at which vowels are sung. They say that the Mekon is cornering most of the supplies of something. Sondar guesses that the Mekon is planning an attack on Venus, Earth or both. Dan agrees, but says that they must rescue Digby and Urb before they do anything else.
A Mecurian takes Dan, Sondar, Peabody and Willie to the Mekon's hideout. En route, they are attacked by Treen fighters. The Mekon only wants the spring-powered car derailed, and its occupants taken alive. But the captive Digby and Urb wake up. They briefly seize the Mekon, just long enough for Urb to call off the attack and smash the televiewer.
The others arrive at the Mekon's lair. They are attacked by two Treens, but overcome them with para-gas. Dan decides to explore with Sondar, and tells Peabody to take Willie and the Mercurian and follow the Treens when they wake up.
Dan and Sondar cross a bridge over a deep chasm to a dead-end ledge, whereupon the bridge swings away, trapping them. A door opens in the wall, and behind it Dan sees D'Arcy, the pilot of the Kingfisher which blew up on the first Venus mission. They go through to meet him, and the door slams shut.
D'Arcy denies working for the Mekon, and insists that the Treens on Mercury are anti-Mekon refugees. When the Kingfisher blew up, he and his men bailed out and were thrown in prison alongside rebel Treens. Later, Treens and Earthmen escaped to Mercury. D'Arcy points out that the Mercurians cannot pronounce the word Mekon, but merely describe a Treen leader. The Treen leader, he says, is called Mistag. The Mekon orders Mistag to open the doors to the room, whereupon Treens burst in and seize Dan and Sondar.
Peabody and the Mercurian lose track of the Treens, but find the bridge, swung away from the ledge, but cross the chasm using some rope. The Mekon tries to stop them getting into his headquarters by slamming the door shut, but the Mercurian holds it open. Because of his strength, Peabody decides to call him Samson.
Dan and Sondar are taken to a spaceship hangar, where D'Arcy explains that he had thought Sondar was a spy from the Mekon. Dan demands that Digby and Urb be handed over, and, when Mistag arrives, D'Arcy says that Dan and Sondar can be allies against the Mekon.
Meanwhile, the Mekon has tried to persuade Digby and Urb to go to Dan, deny having seen him on Mercury, and persuade him to take a ship back to Venus. Suspecting a trick, they refuse, and the Mekon orders that they be put to death.
Mistag tells Dan that Digby and Urb tried to escape, and went through a door that led straight outside without helmets. Perhaps they forgot that Mercury has hardly any atmosphere. He offers to show their remains.
Meanwhile, Willie picks up a scent and leads Peabody and Samson after it. They arrive just in time to see some Treens pushing Digby and Urb into an airlock. Samson turns off the airlock, just as Dan, Sondar, D'Arcy and Mistag arrive. Mistag denies to Peabody that the Mekon is on Mercury, but she reminds Dan of what the guard said earlier on. Realising his mistake, Dan fights to get Digby and Urb out of the airlock, and, realising his, D'Arcy helps him. This works, and the reunited crew return to the hangar.
D'Arcy calls his men, and all the Earthpeople, with Sondar, Urb and Samson, get into a ship called Hermes. They take off in her, but the Mekon fires a demagnetiser at her, and, without power, she goes into orbit. Dan explains that he had expected this. He knows that the Mekon is planning an attack, but wants time to work out what it will involve. What is he gathering, why does he want Dan alive, and why did he trick D'Arcy?
Dan sets Hermes on a course around Mercury's temperate belt, where it is always evening, and where the Mercurians, and presumably the Treens, live, telling Peabody to use a bomb aimer to examine the surface. Digby tells Dan that the Mekon wanted a him to fly to Venus.
Sondar suggests a theory. In order to conquer the Solar System, the Mekon would need to first knock out Venus and Earth in a single blow. For this, he would need a devastating new weapon that could be carried in one space ship, triggered from long range. But no Treen could fly such a mission unchallenged. Only a Theron or Earthman could fly it, and D'Arcy was originally intended for it, but, given his long absence, somebody might have suspected. Nobody, though, would challenge Dan. D'Arcy confirms that he was booked for a special mission, and knows that the Treens are working on a secret weapon.
Meanwhile, on the bomb-aimer, a city comes into view. This is the main Mercurian city, which Samsom calls "Ray-Law", and behind it is a huge domed Treen city. D'Arcy guesses that it is "Vilgoth", the main Treen research centre. There, Mercurian slaves are bringing supplies of a vegetable called fay-saw. D'Arcy recognises it. The Mercurians eat fay-saw, but, when the Treens first tried it, it proved to be a deadly poison to them. As to what the Mekon is doing with the fay-saw, Dan says that they will have to break into Vilgoth to find out.
In the meantime, the Mekon decides to try starving Dan into surrender. For days, Hermes orbits Mercury, and the Earthpeople grow weak. Sondar and Urb are on the point of surrender when a hovercar from the Mekon arrives with food as a bribe for a surrender. Samson and Dan capture the hovercar, and so feed the crew. Dan and Digby then attempt to use it to tow Hermes out of orbit. This works, and, once landed, Samson takes Dan and Digby to Ray-Law. En-route, their Mercurian "springbikes" are attacked by a Treen Thermite shell gun, and they escape into the Ray-Law transport centre, where Samson provides them with disguises.
Back at the hovercar, Sondar sends a faked report to prevent the Treens from getting suspicious. He claims to the Treens that Dan and Digby have surrendered and are on their way.
Dan and Digby's disguises do not work, and they are captured and taken to Vilgoth. There, however, the chief Treen scientist, Garlok, believing them to have surrendered, tells them about the weapon they are expected to fly to Venus.
"Panthanaton" is an aeriform elastic fluid, extracted from the essence of fay-saw. It mixes easily with air, and means instant death to all forms of life that encounter it, as Garlok demonstrates with a chamber of Venusian life forms. Dan is to fly bombs whose warheads contain brittle containers of Panthanaton. Garlok demonstrates a curious feature of Panthanaton -- it will not fill a vacuum, but will always move against gravity in such conditions.
Meanwhile, Sondar, Peabody, D'Arcy, Urb and Willie travel to Vilgoth in the repaired hovercar. At his HQ, the Mekon watches the hovercar take off and realises the "surrender" was false. He arranges to have the hovercar intercepted, and travels to Vilgoth by telesender. There, he tells Garlok that Dan has not surrendered, and tells Dan that he must now surrender. The crew of the hovercar are now in a chamber into which he will release panthanaton if Dan will not comply.
Dan punches a Treen guard and makes a break, but only as far as the panthanaton containers. He seizes one, and threatens to smash it and destroy all life on Vilgoth. The Mekon releases his prisoners, and Dan sends all but Sondar to Ray-Law to find Samson and tell the Mercurians of the Treens' surrender.
Dan and Sondar take the Mekon to the Vilgoth communications centre, where Dan finds a teleradio set powerful enough to contact space fleet HQ on Earth. He tells Sir Hubert that he survived the destruction of the Red Moon, and of the situation on Mercury, but, while he does so, the Mekon escapes and takes a telesender back to his HQ. The set goes dead, and Sir Hubert immediately organises a fleet to Mercury. But it will take two weeks to get there.
Dan and Sondar escape to Ray-Law, where they find their friends addressing a mass rally of Mercurians. Dan tries to persuade the Mercurians to fight the Mekon, but the Mercurians, all pacifists, are reluctant to fight. The Mekon decides on a pre-emptive strike, and orders an air-raid on the rally. This infuriates the Mercurians, who use springbikes to seize Treens' ships with their bare hands.
As battle rages, Dan sends Samson to get Mercurians to salvage weapons from the crashed Treen ships, D'Arcy to the Hermes to pick up his men and the weapons there, Digby and Urb to guard the transport center, and tells Peabody to guard stay and pass orders to the Mercurians. He takes Sondar with him on his own mission.
Remembering the thermite shells he was attacked with, Dan realises their gun is the key to the battle. While Sondar and some Mercurians distract the Treen gunners with springbikes, Dan climbs to the gun and seizes it from behind. The gun captured, Sondar turns it on the dome of Vilgoth. The thermite shells burn through the dome, and the compressed air escapes, causing an explosion that destroys Vilgoth and everything in it. Since Mercury has hardly any atmosphere, the panthanaton drifts harmlessly into space.
Dan gathers an army of Mercurians and leads them on a journey by land to the Mekon's HQ, which he had built as far away from Vilgoth and the panthanaton as possible. When, days later, they arrive, they discover that the Mekon has abandoned his plans, and watch as his fleet takes off. A few seconds later, an explosion destroys the Mekon's HQ, the last Treen settlement on Mercury. A few days later, the ships from Earth arrive to help the Mercurians rebuild Ray-Law, and take Dan and his crew home.
The Red Moon Mystery
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