Sven Bronson, a South African astronomer, discovers that a pair of rogue planets, Bronson Alpha and Bronson Beta, will soon enter the solar system. In eight months, they will pass close enough to cause catastrophic damage. Sixteen months later, after swinging around the Sun, Bronson Alpha will return to pulverize the Earth and leave. It is hoped that Bronson Beta will remain and assume a stable orbit.
Scientists led by Cole Hendron work desperately to build an atomic rocket to transport enough people, animals and equipment to Bronson Beta in an attempt to save the human race. Various countries attempt the same thing. Nations including the United States evacuate their coastal regions in preparation for the Bronson bodies' first pass. As the planets approach, observers see through their telescopes what everyone agrees are cities on Bronson Beta. Tidal waves sweep inland at a height of 750 feet (230 m), volcanic eruptions and earthquakes take their deadly toll, and the weather runs wild for more than two days. As a token of things to come, Bronson Alpha grazes and destroys the Moon.
Three men take a floatplane to check out conditions across the United States and meet with the President. All three are wounded fighting off a mob at their last stop, but manage to return with a precious sample of extremely heat-resistant metal one of them had noticed, solving the last remaining engineering obstacle: No material had been found to make rocket tubes capable of withstanding the heat of the atomic exhaust for very long.
Five months before the end, desperate mobs attack the camp, killing over half of Hendron's people before they are defeated. With the rocket tube breakthrough, the survivors are able to build a second, larger ship that can carry everyone left alive (originally there was room for only 100 of the roughly thousand people Hendron had recruited). The two American ships take off, but lose contact with each other. Other ships are seen launching from Europe; the French ship's tubes melt, causing it to crash. The original American ship makes a successful landing, but it is unknown if anyone else made it. The survivors find that Beta is habitable.
