What started out as an innocent campus prank at Space Academy almost cost the Polaris unit the chance to become officers in the Solar Guard. For punishment, Tom, Roger, and Astro were assigned to spend all their spare time on guard duty in the vicinity of a new building which housed the latest hush-hush project so important to the solar universe. Here they were able to observe some suspicious events which looked to them to be attempts to sabotage a plan to deliver cargo from freighter spaceships to the various planets by means of projectiles shot from the mother ships. Should the plan prove successful, it meant the doom of the present methods of transportation in space. The saboteurs were ruthless, and for a long time the Solar Guard played into their hands. How the three cadets, through their courage and ingenuity, were able to accomplish the downfall of the plotters makes thrilling and exciting reading. Their gallant work not only saves the project but restores their good standing in the Academy. Here is a rousing story which no red-blooded boy will want to miss!