Son of the Tree begins with the arrival of Joe Smith on the planet Kyril, so distant that Earth is but a myth. Kyril is dominated by a religious aristocracy called "Druids", who rule over the five billon commoner "Laity", and who control worship of the "Tree of Life" – a huge tree with a trunk five miles in diameter, and height of twelve miles. The Druids are xenophobic, and consider Joe to be a spy. For unknown reasons, he is befriended by Hableyat, a native of the world of Mangtse, self-admitted spy who finds him a job as a chauffer for Druid Princess Elfane.
After witnessing a murder committed by Princess Elfane's lover Manaolo, Joe Smith flees Kyril on the spaceship Belsaurion bound for the world Ballenkarch, his original destination – only to find that his fellow passengers include Hableyat, Manaolo and Princess Elfane, and that he is caught up as a pawn in a complex three-way political plot between the opposing worlds. Surviving a couple of murder attempts and puzzling over the intentions of Hableyat and Princess Elfane, he arrives on Ballenkarch, where he finds to his surprise that the earthman he was seeking has made himself ruling prince, with the woman he left behind on Earth as his princess. However, his biggest surprise is yet to come, when he discovers the horrific true nature of the so-called "Tree of Life".