"The Summer Tree" is a fantasy novel by Guy Gavriel Kay, published in 1984. It is the first book in the Fionavar Tapestry trilogy, which explores themes of mythology, magic, and the interconnectedness of worlds. The story follows five university students who are transported to the magical land of Fionavar, where they become embroiled in an epic struggle between good and evil. The narrative weaves together elements of Celtic mythology and original fantasy, featuring a rich cast of characters, including King Ailell and the Lady of the Lake. As the protagonists navigate their new reality, they confront their destinies and the impact of their choices on both Fionavar and their own world.