War or peace in the galaxy depended on whether the Sirians, bent on galaxy-control, could be prevented from learning any more about Agrav research. As a member of the Council of Science, Lucky had reason to suspect that a spy was at work on Jupiter Nine. Lucky encounters little cooperation from Commander Donahue, head of Agrav research, who has lost control of his men. The situation looked bleak indeed when the V-frog, which Lucky depended upon for telepathic messages, was mysteriously murdered. Lucky engages in more of the interplanetary razzle-dazzle which have distinguished his earlier career in fighting the pirates of the asteroids, etc., before he discovers that the spy is actually the robot-dog of the blind man, Norrich. A jaunt to Jupiter for a defined market.