The story begins on the New Chicago Monorail Terminal where three members of the Polaris unit, command cadet Tom Corbett, Roger Manning, and Astro, are returning from leave to Space Academy. During the journey, they cross paths with two rude passengers, Wallace and Simms, setting up a rivalry that will persist throughout their upcoming assignment. Upon reporting to their instructor, Captain Steve Strong, they learn they are assigned to take their rocket cruiser, the Polaris, to the upcoming Solar Exposition on Venus as an educational exhibit.
Arriving on Venus, the cadets meet Exposition Commissioner Mike Hawks and encounter Wallace and Simms again, who have secured a concession to operate an old chemical-burning freighter for passenger joy rides in space. The exposition opens grandly with Captain Strong delivering a dedication speech as a time capsule is lowered into the ground. However, strange occurrences soon disrupt the fair, including mysterious clouds of Venusian dirt appearing high in the atmosphere above the exposition grounds.
Suspecting that Wallace and Simms are using their passenger rides as a cover for smuggling operations, the cadets investigate their activities. Their suspicions are confirmed when an underground shaft is discovered beneath the suspects' shack, revealing that the time capsule containing the accumulated technological and historical knowledge of the Solar Alliance has been stolen and torn open. The thieves have taken an information spool detailing an adjustable light-key capable of opening any energy vault across the solar system.
Captain Strong and the cadets embark on undercover assignments as merchant spacemen, traveling from Luna City to Marsopolis in search of the criminals. Through underworld contacts, they learn that Wallace is associated with Bull Coxine, a notorious mutineer and criminal mastermind who previously escaped from a maximum-security prison asteroid. The pursuit intensifies as the stolen light-key is used to raid Solar Guard armories and transport ships for advanced weaponry and military hardware.
The climax unfolds when Coxine launches a devastating surprise attack on the prison asteroid using an atomic-powered ship, freeing all the inmates and building an armed armada for a larger assault. Tom sneaks aboard the pirate ship disguised as an escaping prisoner in order to plant a radar interference beacon. Despite being captured and enduring severe interrogation, Tom stalls long enough for the beacon to guide Captain Strong and the unified fleet of the Solar Guard to the pirates.
The narrative culminates in a massive space battle near Ganymede where the Solar Guard forces confront the pirate armada. After intense dogfights and a desperate struggle in which Astro defeats Coxine in hand-to-hand combat, the criminals are subdued. The adjustable light-key is recovered, the stolen pay roll is secured, and the solar system is finally freed from the threat of space piracy.
