The fifth book in Isaac Asimov's Lucky Starr series. The Council of Science suspects that there is a Sirian spy on a ship using the new Agrav technology, and sends Lucky to find out who it is. Lucky takes along a Venusian V-frog which has the ability to sense, transmit and change emotions. Soon the V-frog is killed. Lucky deduces that only a robot spy would be able to kill it, and spends the rest of the book trying to tell who is the robot.