Disease, death, tyranny, betrayal, and rebellion form the backdrop of the story, against which are contrasted platonic love, loyalty, sacrifice, hope, courage, and pacifism.

The introduction of the story depicts a small apartment in an impoverished area of Sweden, where Skorpan is bedridden with tuberculosis and, the reader is given to understand, unlikely to survive.

The brothers experience adventures: together with a resistance group, they join the struggle against the tyrant Tengil, who rules with the aid of the fearsome fire-breathing dragon, Katla.

To older readers, the book is deliberately ambiguous about how much of the plot is in fact real, keeping open a possibility that it represents Skorpan's fever dreams and imagination, based on the hopeful stories told to him by his brother.

In an unnamed Swedish city, ten year-old Karl Lejon has found out that he is going to die from an unspecified pulmonary disease (most likely tuberculosis).

Karl is introduced to the denizens of the valley, particularly Sofia the dove-keeper, Hubert the hunter, and Jossi, a landlord, and assumes the surname Lionheart along with his brother.

Prompted by a nightmare in which he sees Jonatan in danger, Karl follows him in the middle of the night, but while hiding in a cave, he witnesses a clandestine exchange between two of Tengil's soldiers and Jossi, who has turned traitor to his people.

The Lionheart Brothers soon depart for Karmanjaka and manage to release Orvar moments before he is to be collected and fed to Katla, but their escape is soon discovered.

Karl throws himself off the horse and hides so that Jonatan and Orvar can escape, but soon afterwards he encounters Sofia and Hubert, who are being led into a trap by Jossi.

The Lionheart Brothers barely escape with their lives when an ancient lindworm, Karm, suddenly rises from the waters and engages Katla in mortal combat, which ends with the two monsters killing each other.

A train trip along the lake Fryken, south of Torsby, on a winter's day in 1972, displayed a fantastic dawn which gave her the impulse to write of a land far away.

During a visit at a cemetery in Vimmerby, Lindgren was caught by an iron cross with the text Here rest the young brothers Johan Magnus and Achates Phalen, dead 1860.

Another inspiration was when, during a press conference for the film of Emil i Lönneberga in 1971, she saw how the young lead character actor Jan Ohlsson got in the lap of his older brother Dick.

[3] The contrasts, the evocative storyline and the themes of yearning for comfort, of brotherly affection, loyalty and struggle for freedom went over well with a wide readership that was often familiar with Tolkien's The Lord of the Rings and with folktales, and in many ways Lindgren's novel is an example of what Tolkien described as inspiration drawn from "the deeper folktale" (in On Fairy-Stories) and the cathartic, poignant power of such stories.

In 2007, the book was adapted into a musical by Bo Wastesson[4] (music), Staffan Götestam (manuscript - coincidentally, Staffan played Jonatan in the 1977 film adaptation) and Ture Rangstrom (lyrics), directed by Elisabet Ljungar[5] at the Gothenburg 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.