A space team consisting of pilot Buzz Lloyd, scientist Clayton Stone, and mission commander Jim Pruett tests man's endurance in outer space. Observing that the crew's efficiency has faltered after five months, Houston supervisor Charles Keith orders its return. Upon reentry, however, the ship malfunctions, leaving the astronauts with no means of reaching earth. Veteran astronaut Ted Dougherty and a presidential mandate compel the reluctant Keith to mount a rescue effort. As the astronauts' oxygen runs out, the rescue mission races against time. When the launching of Dougherty's spaceship from Cape Kennedy is delayed by a hurricane, the astronauts' wives are brought to Houston for televised conversations with their husbands. During his interview Lloyd becomes hysterical, in so doing consuming additional oxygen. To compensate for this loss Keith suggests a sacrificial suicide, a possibility which the team rejects. Leaving the vehicle to correct a mechanical defect, Pruett rips his suit and floats away in space. Although the oxygen has dangerously diminished, Stone donates his supply to Lloyd, breathing the little left inside the ship's cabin. Panicking, Lloyd opens the hatch, depleting Stone's precious store. As hope dims, a Russian spaceship arrives with sufficient oxygen to maintain the American astronauts.