Mission control on Earth tracks MR-1 as it returns from the first crewed mission to Mars, slipping into orbit with a tone of disbelief—the technicians had expected the ship to be lost, or worse. In a tense debrief, it becomes clear that only two of the original four crew members are alive when the craft is guided down to a safe Nevada landing. The survivors are Dr. Iris Ryan and Col. Tom O'Bannion, the latter's entire right arm swollen and green with an unnatural growth. The shocking discovery of the arm's condition—an alien infection of sorts—frames the rest of the story as a medical mystery and a planetary mystery at once.
Back on Earth, Dr. Iris Ryan recounts the voyage and the crew’s experiences on the red planet in vivid, retrospective detail, as if the events are happening again in real time. Her testimony doubles as a field report and a medical narrative: she helps to seek a cure for Col. O'Bannion’s infected arm while the team on Earth processes what happened and what it might mean for future missions. The tone remains clinical, but the implications are fearsome, hinting at a broader significance to humanity’s push farther into the solar system.
Once the MR-1 reaches the Martian surface, the crew encounters dangers that test both their body and their wits. A carnivorous plant attacks, and Chief Warrant Officer Jacobs uses his freeze-ray—fondly nicknamed “Cleo”—to kill it and keep the team alive. Not long after, they confront a colossal creature that looks like a bizarre cross between a rat, a bat, a spider, and a crab. Its presence is initially misread as a forest of tentacle-like structures, and Jacobs again proves instrumental in repelling the threat with the same trusty weapon.
The dangers multiply as the crew attempts a return to their ship. It soon becomes apparent that they are being deliberately blocked from escaping Mars by a mysterious force field, severing their radio contact with Earth and trapping MR-1 on the planet’s surface. In a tense exploration of a Martian lake, they glimpse a distant city with towering structures across the water, a sight that fills them with both awe and dread. As they attempt to cross, a giant amoeba-like being with a single spinning eye closes in, and Jacobs is drawn into its gelatinous body, ultimately consumed. The creature also infects Col. O'Bannion’s arm, adding a personal fear to the already dangerous situation.
In a bid to survive, O'Bannion leads the crew to rewire the ship and electrify the outer hull, driving the creature away and enabling them to lift off. The ascent proves physically and emotionally taxing, and the MR-1’s designer, Professor Gettell, dies of a heart attack amid the extreme strain. Despite the odds, the three remaining crew members push MR-1 back toward Earth as the force field that trapped them seems to deactivate.
Upon return, the medical team cures O'Bannion’s arm with electric shocks, and Earth-bound technicians review the expedition’s data recorders. A recording from an alien voice reveals that the Mars mission was allowed to depart with a warning for humanity. The message explains that “We of Mars have been observing human development on Earth for many thousands of years and have determined that humanity's technology has far outpaced progress in cultural advancement.” The voice condemns humanity’s incursion into Mars and issues a stern warning: if future missions ever return, Earth will face retaliation and destruction.
