This fourth and possibly concluding volume in Pohl's popular Heechee Saga takes place 100 years after the first, Gateway, when the discovery of abandoned alien spaceships opened the galaxy to human exploration. Now allies, human and Heechee, are guarding against the Foe, a race of energy beings that has already wiped out several civilizations. Millionaire Robin Brodhead, who has survived his physical death to become a computer-stored intelligence, tracks suspicious Foe-directed transmissions back to a Pacific island and the refugee children who have unwittingly carried the Foe to Earth. Robin's first contact with these beings becomes vital to human and Heechee existence. Although Pohl relies rather heavily on cute aliens and wise-guy computer programs, his restless, loquacious narrator Robin whips the reader from planet to planet and from past to future, rapidly solving problems and just as rapidly shoveling up new ones in the best Pohl fashion.