The two main characters are two brothers; the older Jonatan and the younger Karl. The two brothers' surname was originally Lion, but they are generally known as Lionheart. Karl's nickname is Skorpan (Rusky) since Jonatan likes these typical Swedish toasts or crusts.
In Nangijala, a land in "the campfires and storytelling days", the brothers experience adventures. Together with a resistance group they lead the struggle against the evil Tengil, who rules with the aid of the fearsome dragon Katla and her firebreath.
Some time later a fire spreads throughout their home. Jonatan attempts to rescue his little brother by carrying him on his back and jumping out the window. Jonatan is injured in the fall and dies. Karl is left alone, and starts to wonder if the story about Nangijala is really true. A white pigeon appears one night on his window sill, and Karl interprets it as a confirmation that it is.
Two months after his brother Jonatan, Karl dies from his illness. Right before he dies, he leaves a message for his mother: "Don't cry mommy, we'll see each other again in Nangijala". Immediately after his death he finds himself standing outside a small cottage. He is no longer sick, and runs down to a river, where his brother is sitting and fishing. Jonatan tells him that they will be living at the Riders farm in the Cherry Valley. They each have a horse, Grim and Fjalar, and a short time of peace and joy eludes, with fast riding and discovery expeditions.
Karl meets Sofia, whose rose garden Jonatan tends to. He is informed in small pieces that there are problems even in Nangijala. On the other side of the mountains lies Törnrosdalen (the Thorn Rose Valley) which has been occupied by the evil Tengil, who has descended from the country of Karmanjaka with his men and built a wall around it. He has enslaved the original inhabitants. With the dragon Katla at his service, he appears unbeatable. Jonatan does not wish to tell Karl about Katla, since he fears the information would frighten him.
The people of the Cherry valley, lead by Sofia, helps the resistance movement in the Thorn Rose Valley, but they know a traitor exists in the village. Someone from the Cherry Valley is helping Tengil, as Sofia's white doves, which flie with secret messages between the valleys, are being shot down with a bow and arrow.
One day Jonatan leaves for the Thorn Rose valley, where the resistance leader Orvar has been arrested and sits in custody in the Katla cave. His sense of duty makes him go, igoring the dangers. Karl is left alone, and after a few days he attempts to follow his brother. One night he hides in a cave, and two of Tengil's men stand outside it to meet the traitor.
In the morning, Karl is discovered by the two soldiers. They are suspicious and bring him to the Thorn Rose valley. Karl tells them he lives with his grandfather, and the soldiers demand that he show them his house and his grandfather. Luckily an old man is standing outside a small house with a white dove, and Karl throws himself into the old man's arms. The soldiers are satisfied. The old man, Mattias, is also part of the resistance movement, and inside the house Karl finds Jonatan asleep.
A happy reunion ensues when the two brothers meet again. Mattias' house lies right next to the high wall, and there are constantly guards from Tengil snooping around to see if someone is doing something forbidden. Jonatan is digging an underground tunnel which will go from Mattias' house, under the high wall and end in a forest on the inside.
When Tengil himself shows up in the Thorn Rose valley, everyone has gathered in the square. He is dressed in black, rides on a black horse and looks cruel. All the men in the village have to get in a line and Tengil picks out the ones who will be brought to Karmanjaka to carry rocks. In Karmanjaka Tengil is building an impenetrable fort so that he will never be conquered. A man who protests is quickly incapacitated by Tengil's soldiers.
Jonatan has almost completed the underground tunnel. The brothers manage to escape the valley. As they stop to bathe in the river, they must hide from groups of soldiers. One of them rides out in the streaming river to show his bravery, but almost drowns. Jonatan shows empathy with the enemy soldier by saving him and his horse from drowning.
When they sit down to camp at the Karma Falls, Karl gets to see Katla (a firebreathing female dragon) for the first time, the dragon that Tengil uses to terrorize the people. Tengil controls Katla with the help of a trumpet.
The next day the brothers cross the river, using the suspension bridge that connects the Thorn Rose Valley from Karmanjaka. The entrance to the Katla cave is guarded by Tengil's soldiers, but Jonatan manages to find a second entrance. Deep in the mountain they arrive at the Katla cave where Orvar is kept. They manage to release Orvar from his wooden cage, but his escape is soon discovered. They ride back as fast as they can towards the Karma Falls and the bridge, but the pursuing soldiers start overtaking Karl and Jonatan, who are both riding on Grim. Karl throws himself off the horse and hides in a ditch so that Jonatan can escape.
When the pursuing soldiers have gone away, Karl moves on to the place where they went swimming, and hides in a tree. At dusk three familiar people show up: Sofia, Hubert and the traitor Jossi. When Karl tells Sofia that Jossi is the traitor, she gets angry. Once Jossi's shirt is forced off, and everyone can see the "Tengil's mark" on his chest, they believe Karl. Jossi throws himself into a small boat, but the stream catches him and brings him to certain death in the waterfall.
Shortly thereafter, the people of the Thorn Rose Valley rise up against Tengil and his men. Once Tengil shows up with Katla, all seems lost, but Jonatan manages to pull the horn out of his hands. As Katla no longer obeys the tyrant, the dragon attacks Tengil and his men. Reluctantly, Katla becomes under the tenuous control of Jonatan.
Once the fight is won, Orvar asks Jonatan to bring Katla back to the Katla cave. When Jonatan and Karl rides over the suspension bridge, their horse Grim gets frightened and Jonatan drops the trumpet down into the river. Katla then chases them up the mountain. They finally hide high up on a cliff where Jonatan pushes a big rock down on Katla. Katla falls backwards into the river, and a fight breaks out between Katla and the (male) lindworm Karm. The two beasts, who have been waiting for this battle since the beginning of times, savagely kill each other.
Jonatan and Karl sets up a camp and Jonatan explains that during the fight he was burned by Katla's fire, and that he now will soon become totally paralysed and then die. Jonatan would rather die immediately in a land that awaits beyond: he tells Karl about the land Nangilima, where there are only happy adventures. Karl does not want to separate again from his brother but carries him on his back out to a cliffdrop and jumps. His last words at the bottom of the abyss are: "Oh, Nangilima! Yes, Jonatan, yes - I see the light! I see the light!"
To ease Karls death anxieties his brother tells him the story about life after death, where people end up in Nangijala — a pre-technology, medieval realm "at the other side of the stars".
There are three areas mentioned of Nangijala: the Cherry Valley, the Thorne Rose Valley, and Karmanjaka.
The Cherry Valley is a paradise that has everything a 10-year-old may wish for: plenty of fish, rabbits, and your very own horse. Here Karl can do everything he could not do on earth — run, swim and gallop. The people of the Cherry Valley live in a barter economy where everything is free and everyone helps each other: Jonatan is a gardener for Sofia, who in turn gives the boys food. Karl's initiation into this society is symbolized by his change of clothes, to a more medieval style, lest the rest would think he is strange.
On the other side of the hills is the Thorn Rose Valley, occupied by Tengil and his men. The inhabitants of the valley have been forced to build a high wall around it with only two gates, one toward the mountains and one toward the river. Violence rules here and the inhabitants are forced to work like slaves. No one gets in our out without knowing the password which is,"All power to Tengil, our liberator".
From the Thorn Rose Valley, the Ancient River flows towards the Ancient Mountains where the dark and dreary Karmanjaka lies, and where Tengil has built his castle. The dragon Katla lives in Karmanjaka.
In Sweden the phrase "See you in Nangijala" is some times used in obituaries when children have died.
The character Tengil makes Karl scared, but he is inspired by his brother to overcome his fear. The first time is when he leaves the Cherry valley, the second time is when he jumps off the horse, when he and Jonatan is being chased, and the third time is when he jumps with his brother of the cliff, to go to Nangilima. The last death jump shows that he repeats Jonatan's heroic act in the beginning of the book, and finally becomes a "true" Lionheart.
The character Karl is played by Lars Söderdahl in the movie.
Jonatan is a hero but not a rebel fighter, since he is a pacifist. On the journey towards Karmanjaka he rescues one of Tengil's soldiers from being swept away by the hard river currents, even though he is risking arrest in doing so. The day before the big uproar he explains to Orvar that he can not kill, not even to save his own life.
Jonatan is played by Staffan Götestam in the movie.
In the movie Sofia is played by Gunn Wållgren.
In the movie Jossi is played by Folke Hjort.
For the recording of the movie in 1977 a model of Katla was manufactured at Pinewood studios outside of London. Inside the model, which was 4 metres high and 8 metres long, there was room for three people; one at each front leg and a person at a small control table. The model could blink, breathe fire and whip its tail. The model cost 100,000 Swedish kronor to manufacture. The director Olle Hellbom thought that it was not scary enough originally, and made it look filthier on the surface.
The movie was the only appearance of the mechanical monster. As of 2004 the model is stored at the film company Dekor in "Trollywood".
We meet Tengil for the first time when he sits atop a black horse in the square of the Thorn Rose Valley. He is dressed in a black hood and has a chestplate with a red flame on it. He wears a black helmet with a purple plume. Tengil's power is enforced by his soldiers and by his control of the dragon Katla.
At the end of the story, in the fight between Tengil's soldiers and the people of the Thorn Rose Valley, Tengil's powers appear to have been defeated. Suddenly Katla shows up and the situation turns to Tengil's advantage. Jonatan appears to engage Tengil in a holmgang, but he targets rather Tengil's trumpet. Losing the control of Katla, which passes temporarily to Jonatan, Tengil dies under the fire of his own dragon.
In the movie Tengil was played by Georg Årlin.
The origins of the book have been described by Lindgren several times. A train trip along the lake Fryken, south of Torsby, on a winderday in 1972, displayed a fantastic dawn which gave her the impulse to write of a land far away. "It was one of those fantastic mornings with pink light over the lake - yes, it was something of unearthly beauty, and I suddenly got a strong experience, a sort of vision of the dawning light of humanity, and I felt something lit inside. This may turn into something, I thought".
During a visit at a cemetery in Vimmerby, Lindgren was caught by an iron cross with the text Here rest the fragile brothers Johan Magnus and Achates Phalen, dead 1860.It gave her the inspiration to write a story with two young brothers and death: "Then I know suddenly that my next book would be about death and about these two small brothers."
Other critics believed that Lindgren painted the tale in a very black and white world:
On the other hand readers reacted largely positive: "It is clear that children had a great wish for tales and preferably these kind of exciting tales. Right now I am swamped with letters from children - from several countries - that loves the Brothers Lionheart. Never before have I received such a strong and spontaneous reaction on any book." - Letter written by Lindgren in 1975.
In 2007 the book was adapted into a musical by Bo Wastesson (music), Staffan Gotestam (manuscript) and Ture Rangstrom (lyrics), directed by Elisabet Ljungar at the Goteborg Opera House in Sweden, with the leading parts played by Hanna Brehmer (Skorpan), Alexander Lycke (Jonathan) and Annica Edstam (Sofia), orchestra conducted by Marit Strindlund, choreography created by Camilla Ekelof, costume and stage design by Mathias Clason. The musical opened on March 3 2007.