A stranger calls at the home of divorcée Anna Preston, asking to collect some of her ex-husband's belongings. Anna agrees, but is suspicious, and tries to contact her 'ex' at the space-research establishment where he worked. But none of his colleagues can tell her where he is and the authorities block all her efforts to find out. Anna grows more convinced that he is in danger, but the truth is more horrifying than even she can imagine.

Dr Andrew Cook, inventor of a process for making living replicas of human beings, has unwittingly duplicated himself and becomes a secret weapon in the Solar System's struggle for economic survival against a powerful alien culture, the Sentients.