Theophilus Merlan, a shy geologist, courageously saves an alien boy's life on the planet Yellowsand and acquires some of his DNA. Since the boy is royalty and the present, dying Speaker admires Merlan, he tells Merlan that he is now part Shakk himself and must be the next Speaker or ruler of the Shakk people. Hour by hour, Merlan finds himself changing mentally and physically from human to Shakk. Even more terrifying is the Speaker's revelation that the Shakk's two vile and vicious alien enemies, the Xanteans and the Merotox, are about to visit Yellowsand.

These invaders seek to conquer Yellowstone and enslave the Shakk, and only Merlan can save his new people. But to do so, he has to use all his wits and courage. The situation is complicated further by the threat of the Speaker's grandson, who hates Merlan and seeks to kill him because the Speaker has chosen a human to be the next Speaker in his place. Merlan finds he must do more than just hold onto his sanity and adjust to his changing abilities and perceptions. He must also be a father to an alien boy and cope with his growing love for Ann Benson, the mission's navigator. As he becomes increasingly alien, Merlan wonders if they have any chance at happiness. His task of saving the Shakk race seems hopeless, yet he feels he must try even if it costs him his life.