The first Agrav ship ever built was named Jovian Moon. It would bring the stars into man's reach, but Sirian sabotage threatened to snatch the prize from Earth.

Lucky Starr, officer of the Council of Science, had a job to do: find the traitor who was transmitting the information to the Federation's Sirian enemies.

Nearly a thousand men made up the personnel of the project. But one of those men was a Sirian robot spy -- a robot spy who heard everything, saw everything, knew everything.