At the shore near Elsinore, he is among the group of resistance fighters trying to cover the escape to Sweden of an important scientist (evidently the nuclear physicist Niels Bohr).

With a German force closing in, Carlsen is shot and suddenly finds himself transported to a parallel universe, a world in which Northern European legend is real.

He finds that the clothes and armor fit him perfectly, and he knows how to use the weapons and ride the horse as well as speak fluently the local language, a very archaic form of French.

They are induced to follow the seemingly attractive elvish Duke Alfric of Faerie, who in fact plots to imprison Holger in Elf Hill, where time runs differently.

They escape and, after encountering a dragon, a giant, and a werewolf, reach the town of Tarnberg, where they are joined by the mysterious Saracen, Carahue, who has been searching for Holger.

The novel is a pastiche of interwoven stories and draws on the corpus of Northern European legends, including Ogier the Dane, the Matter of France, Arthurian romance, Oberon (Duke Alfric in the novel), Germanic mythology, and traditional magic.

It uses related literary sources such as Edmund Spenser's The Faerie Queene, William Shakespeare's A Midsummer Night's Dream, Robert Burns's Tam o' Shanter, and Mark Twain's A Connecticut Yankee in King Arthur's Court.

Holger later appears as a minor character in Anderson's A Midsummer Tempest, in which he is seen in a mysterious "Inn Between the Worlds" after he managed at last to leave the 20th century and to wander the various alternate timelines by using the spells from a medieval grimoire.

At the inn, he encounters Valeria Matuchek, a character from another Anderson book, Operation Chaos, who instructs him in the sophisticated scientific magic of her world and gives him a better chance.

In 2014, Harry Turtledove wrote, as his contribution to Multiverse: Exploring Poul Anderson's Worlds, edited by Greg Bear and Gardner Dozois,[3] a short story, "The Man who Came Late".

Michael Moorcock cited Three Hearts and Three Lions as one of the works that greatly influenced his own fantasy, which is similarly set in a universe in which the forces of Law and Chaos are pitted in an eternal war with each other .

[7] The novel influenced the role-playing game Dungeons & Dragons, especially the original alignment system, which grouped all characters and creatures into "Law" and "Chaos".