Three Hearts and Three Lions is a 1961 fantasy novel by American writer Poul Anderson, expanded from a 1953 novella by Anderson which appeared in Fantasy & Science Fiction magazine.
Plot
Holger Carlsen is an American-trained Danish engineer who joins the Danish resistance to the Nazis in World War II. 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. This world is divided between the forces of Chaos, inhabiting the "Middle World" (which includes Faerie), and the forces of Law based in the human world, which is in turn divided between the Holy Roman Empire and the Saracens. He finds the equipment and horse of a medieval knight waiting for him. The shield is emblazoned with three hearts and three lions. 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.
Seeking to return to his own world, Holger is joined by Alianora, a swan maiden, and Hugi, a dwarf. 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. Holger learns that Morgan Le Fay, his lover in a forgotten past life, is his ultimate adversary.
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. Based on the advice of the wizard, Martinus Trismegistus, they set out to recover the sword Cortana. The sword is in a ruined church, guarded by a nixie, cannibal hillmen, and (most dangerous of all) a troll.
On the perilous quest, Holger and Alianora fall deeply in love with each other. However, Holger avoids physically consummating this love though Alianora wants him to as he intends to return to his 20th-century world. But with the perilous Wild Hunt on their tracks, Holger and Alianora pledge their love and he promises, if surviving the ordeal ahead, always to remain with her. However, the decision would be taken out of his hands.
Once the sword is recovered, Holger discovers that he is the legendary Ogier the Dane, a champion of Law. He vanquishes the forces of Chaos and is transported back to his own world, right back to the battle in Elsinore and, with a burst of superhuman strength, vanquishes the Nazi troops and enables Bohr to escape and play his part in the Manhattan Project. Thus, in two worlds, Holger/Ogier has fulfilled his destiny of fighting evil forces and preserving Denmark and France. The magical forces involved have no consideration for the hero's love life by leaving him stranded away from his beloved Alianora. Desperately wanting to return to the other world, he seeks clues in old books of magic. His enduring affinity with the medieval world in which he met her is expressed by a decision to convert to Catholicism.
Inspiration
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. It also shows influence of J. R. R. Tolkien's The Hobbit and The Lord of the Rings with references to Mirkwood and wargs. It has some similarity to C. S. Lewis's The Lion, the Witch and the Wardrobe.
The dividing line between the Empire in the West and threatening Faerie to the East seems to mirror the Cold War's dividing line between the West and East blocs that ran running through the real Europe at the time of writing.