The story begins as Lieutenant David Corriston approaches the glittering Space Station, a massive artificial moon serving as a central terminal and refueling depot for interplanetary vessels. While in the passenger lounge, he meets a young woman who is secretly Helen Ramsey, the daughter of the ruthless Martian industrialist Stephen Ramsey. Accompanying her is her bodyguard, Jim Clakey, who is abruptly murdered by a poisoned Martian barb before he can expose the gravity of the situation. Corriston soon realizes that dangerous impostors have infiltrated the Station and the ships traveling to and from Mars, using revolutionary prosthetic masks to steal identities and secure control over uranium shipments.
Following the murder, Corriston is knocked unconscious and later finds himself confined in a psychiatric cell on the Station after the ship's captain refuses to believe his warnings. Despite being heavily guarded, Corriston escapes his cell to investigate further, witnessing a freight ship carrying unstable uranium crash into North America. He navigates the complex maintenance corridors and discovers that a syndicate of criminals, led by the ruthless Henley, has replaced the Station's command staff with phonies wearing emotion-sensitive false faces to advance their illegal enterprise against the Martian colonists.
Corriston manages to communicate with the real station commander and uncovers Helen Ramsey trapped within the maintenance levels. Although Henley attempts to eliminate Corriston repeatedly, the plot unravels as an international naval fleet arrives to blockade the Station. Corriston joins forces with local colonists and loyal personnel to track Henley across the harsh Martian desert to his hidden stronghold.
In the final confrontation at Ramsey's fortress, Corriston faces Henley directly. After a tense shootout, Henley is defeated, and it is revealed that Stephen Ramsey utilized a double to bargain with the criminals while he personally led a rescue mission. With the conspiracy shattered and the impostors neutralized, the Station is restored to lawful command, and Corriston looks forward to a future with Helen Ramsey.
