Earth's first expedition to Mars, led by physicist Dr. Lane [John Litel], sets out with Prof. Jackson [Richard Gaines], engineer and spaceship designer Jim Barker [Arthur Franz], and Barker's assistant Carol Stafford [Virginia Huston], who earned her degree in spaceship engineering in just three years. A seasoned war correspondent, journalist Steve Abbott [Cameron Mitchell] joins the crew to chronicle the mission, bringing a keen eye for danger and a steady gaze on the unknown ahead.
When a meteor storm disables their landing gear and knocks out radio contact with Earth, the crew faces a harrowing choice: crash-land on Mars or turn back to Earth. They choose to press on, fully aware that they may never return home, driven by the resolve to complete the historic journey.
Upon landing, they are greeted by five Martians who look human and speak fluent English. Ikron [Morris Ankrum], the president of the Martian planetary council, explains that the Martians learned Earth languages from broadcasts. Their own attempts to transmit to Earth have yielded only faint, unintelligible signals. The Earth crew are taken to a vast underground city sustained by life-support systems powered by a fictional mineral called Corium, from which water and air are drawn and energy generated.
Inside, Tillamar [Robert Barrat], a former president and trusted council advisor, along with Terris [Lucille Barkley], a young Martian woman, guide the explorers to their quarters and arrange automated meals. The Earth crew marvel at Martian technology and quickly seek help repairing their damaged craft. Ikron reveals that their Corium supply is nearly depleted and quietly proposes a plan: reproduce the Earthship, building a fleet that could evacuate the Martians to Earth, thereby saving their species but potentially enslaving humanity. The council votes to pursue this strategy, while also choosing to hold the Earthmen captive during the repairs.
Alita [Marguerite Chapman], a leading Martian scientist, is placed in charge of the spaceship project, and Ikron employs Terris as a spy to monitor progress. Jim, sensing danger, enlists Alita’s help to stage a deliberate explosion aboard the ship to slow the repairs. When the countdown to departure is set for the next day, Jim announces that Tillamar and Alita will join the expedition, with Alita to become his wife, a revelation that unsettles the balance of power.
Terris reports the suspicious conduct to Ikron, prompting the Martians to seize Alita and Tillamar as a precaution. Undeterred, Jim and his allies outmaneuver the guard presence and secure a path to the repaired ship. A brief confrontation with Martian guards at the gangway ends with the three of them boarding, and the expedition finally lifts off, heading toward Earth with the fate of two worlds hanging in the balance.
