Carlsen is an Allied covert operative in World War II who assists the Danish Resistance to the Nazis. After an explosion, he finds himself carried to a parallel universe, which proves to have the Matter of France as its historical past. There he finds that the evil of Faerie is encroaching on humanity. His quest finally leads him to discover that he is Ogier the Dane, sent to this universe by Morgan le Fay, and to fight the battle that drives back the evil. After succeeding in this fight, he is thrust back into our world, and is able to ensure that the Nazis can not stop a crucial escape of Niels Bohr from occupied Europe.
While the magical forces which transported Holger from world to world have let him accomplish his task of saving two worlds, they had no concern for his love-life. He is left separated from Alianora, a swan may with whom he had fallen in love while on his quest in that other world. Anderson's novel ends with Carlsen in the post-war years, searching for a way to return to Alianora.
Holger later appears as a minor character in Anderson's A Midsummer Tempest, which reveals that in 1950 Holger did manage to start traveling between the worlds with the help of the spells from one such grimoire. However, his travels are completely random, putting him in many new dangers. Finally, he arrives at the magical in, The Old Phoenix inn, where he meets Valeria Matuchek (a character from another Anderson novel, Operation Chaos). Her world has applied the scientific method to magic and had far more precise knowledge on traveling between timelines. Valeria instructs Holger in the mathematical formulas of her world. With her help, he is able to know where he is going - but getting back to Alianora's world is still not easy.
Holger Carlsen in "The Man who Came Late"
For thirty years Holger sought Alianora and her world. He finally reached it and began to physically search the land for her. One morning he came out of a small wood and found her by a village well. However, while Alianora still had feelings for him, she had not waited for him and married and had children. She could not honorably return his love.