Four closely related science-fiction telepathic spy novellas. First published in Astounding Science Fiction, July 1949, 1950 and 1951 and Galaxy Science Fiction 1950. Fantastic but true! "Having a Daya-Bal as a Zone Agent makes me extremely uneasy," the Executive Scientist said. "Especially when she's given one of your ultimate space ships." "I can vouch for her," the Galactic Co-ordinator said casually. "But supposing another people—like the Daya-Bals—came into possession of one of our ultimate ships. They could duplicate it." "Don't worry—our Agents are psychologically incapable of giving away secrets that could cause us harm." "I know," the Scientist said. "That's why I was surprised to discover that there are two other Daya-Bals secretly aboard our Agent's ship!" (From the back cover)