An international consortium of scientists, operating as Project Inner Space, is trying to tap into the Earth's geothermal energy by drilling a very deep hole through the Earth's crust into the mantle below. The scientists are foiled by an extremely dense layer of material at the boundary between the two. To penetrate the barrier and reach the magma below, they intend to detonate an atomic device at the bottom of the hole. The leader of the project, Dr. Stephen Sorenson (Dana Andrews), who is (secretly) dying of cancer, believes that the atomic device will burn its way through the barrier, but the project's chief geologist, Dr. Ted Rampion (Kieron Moore), is convinced that the lower layers of the crust have been weakened by decades of underground nuclear tests, and that the detonation could produce a massive crack that would threaten the very existence of Earth. The atomic device is used and Rampion's fears prove justified, as the crust of the Earth develops an enormous crack that progresses rapidly. Sorenson discovers that there was a huge signature of hydrogen underground, which turned the small conventional atomic explosion into a huge thermonuclear one that was millions of times more powerful. Another atomic device is used in the hope of stopping the crack, but it only reverses the crack's direction. Eventually the crack returns to its starting point at the test site, and a huge chunk of the planet outlined by the crack is expected to be thrown out into space. Sorenson remains at the underground control center to record the event despite pleas by his wife Maggie to evacuate with the rest of the project staff. She and Rampion barely escape in time to observe the fiery birth of a second moon.
The film was released only a few years before the theory of plate tectonics became mainstream, and later commonly-accepted fact. As it turned out that the surface of the planet was heavily cracked already, the scenario presented in the film is now considered highly improbable. Additionally, the notion of a chunk of the Earth being thrown off into space makes no scientific sense, since even a chunk of the planet completely separated from the rest by a crack would still remain in its original position due to gravity.