Dr. Stephen Sorenson plans to tap the geothermal energy of the Earth’s interior by means of a thermonuclear device detonated deep within the earth. Despite dire warnings by fellow scientist Ted Rampion, Dr. Sorenson proceeds with the experiment after secretly learning that he is terminally ill. This experiment causes a crack to form and grow within the Earth’s crust, which threatens to split the Earth in two if it is not stopped in time.
Project Inner Space, alias Project Intraterrestrial Energy, is a bold attempt to drill directly into the earth's outer core and set up, in effect, a controlled volcano. The United Nations Economic, Scientific, and Cultural Organization (UNESCO) has agreed to finance the project, the brainchild of Dr. Stephen Sorensen (Dana Andrews), after he has promised unlimited geothermal energy for an energy-hungry world.Dr. Sorensen's wife, Maggie (Janette Scott), guides a convoy of senior UNESCO officials, led by Sir Charles Eggerston (Alexander Knox), to the Project site. Sir Charles orders the convoy to stop as it passes the Project's bore hole complex. He has spotted the real reason for his trip: a ballistic missile, minus its warhead, hanging nozzle-up from a crane, directly over the bore hole. Frostily he demands that Maggie explain the missile; she confidently reminds him that it is not armed, and that he and his fellow Science Councillors are there to decide whether to allow the arming or not.Maggie leads Sir Charles and the others to the main building, built over a two-mile-deep shaft. There they ride an elevator (called a "lift" here) two miles down to the Central Operations complex: essentially a bunker where the real work of Project Inner Space takes place. There, Sorensen waits for the Council–but also is taking an X-ray treatment to a malignant growth (probably malignant melanoma, though this is never made clear) in his hand. His doctor, Bill Evans (Alfred Brown), knows that the treatments will not work, but, out of respect for the doctor-patient confidence that is basic to the Oath of Hippocrates, agrees to keep Sorensen's illness, and its severity, secret from everyone. That includes his wife, who is young enough to have been his daughter. (In fact, Maggie was once a student of his; that's how they met.)Sorensen cordially greets the Council and readily acknowledges a problem that Sir Charles challenged Maggie with on their way down: namely that Sorensen's drilling team has hit a solid wall that is all that separates them from the magma, and that in 17 weeks he hasn't advanced an inch. So now Sorensen wants to blast his way in with a thermonuclear missile (a gift from the United States). Sir Charles asks why the project's chief geologist, Ted Rampian (Kieron Moore), is not present. Rampian is a dissenting voice: he has warned that using a thermonuclear warhead as a demolition charge will deliver such a shock to the earth that the crust will crack. Sorensen gives a dismissive demonstration of Rampian's theory, and then delivers the majority opinion: that the detonation will release great heat and merely melt through to the magma.Sir Charles and the rest of the UNESCO Council are satisfied. But Ted Rampian returns, and is outraged to see the missile not only hanging in place, but now equipped with its warhead! Finding out that the UNESCO Council has already been and gone, without hearing from him, doesn't improve his mood. He angrily resigns from the Project and prepares to take all his notes and charts with him to London to lay his case before Sir Charles directly. Maggie begs him not to go, but she cannot stop him. (As a complication, Ted and Maggie were "an item" before she enrolled in Sorensen's class. Ted is still jealous, and they both know it. Perhaps even Steve knows it.)Rampian does get in to see Sir Charles and set forth his case in detail: years of nuclear testing (before the test ban treaty) have created "fissures" in the crust, several of them leading to a major tectonic fault. A thermonuclear detonation at that depth will crack them open, and if the crack extended along the fault, who knows where it might lead? Sir Charles frantically puts a call through to the Project. But they are too late. Sorensen orders the missile shot to proceed, desperate to achieve success before his funding gets cut and he dies before he can see his work done. The missile takes a dive, the warhead detonates, and magma shoots up to the surface, exactly as Sorensen said it would. Sorensen, generous in victory, asks Sir Charles to tell Rampian to get himself back to the Project and get back to work!Rampian returns to a Project in a triumphant mood. But then the earthquake reports start to trickle in. Rampian realizes that they are right along the fault, as he feared. Hastily he organizes a dive in the Indian Ocean, and there photographs what he was afraid to find: a chasm with magma boiling out of it, extending at three knots along the floor of the Indian Ocean. He brings his photographs back to Sorensen, who acknowledges the horrible truth: he has cracked the earth's crust, and might have started something that could destroy the world.The UNESCO Councillors are outraged, of course, but Sir Charles tells his colleagues, quite reasonably, to let Drs. Sorensen and Rampian work out a way to solve the problem. Rampian returns to the Project and assumes command. He and Sorensen then pore over spectrographic data and films of old nuclear detonations, and finally realize what has happened: the missile touched off a fusion explosion in a layer of hydrogen in a pocket beneath the magma, so that the force of the explosion was greater than Sorensen expected. But this finding also suggests a solution: another detonation in the path of the crack could break a gaping hole in the crust; the crack would run into it and stop. Rampian sets out with a heat-shielded fission bomb and asks a fellow scientist, Steele (Mike Steen), to descend into a dormant volcano crater with him to drop the bomb into the lava. Steele dies when the bomb, catching Steele's line between itself and the surrounding rock, parts the cable and sends Steele falling into the lava pit. Rampian, nearly overcome with grief, barely manages to release the bomb into the lava before he collapses at his line, overcome by the heat. The crew hastily haul him up and take him off the island, to a nearby island where they give the signal to set the bomb off. The bomb blows the volcano cone away, and at the Project site, the seismologists confidently believe that the crack has in fact stopped.Then Dr. Evans sends a message to Maggie informing her of the truth about her husband: that his disease is malignant, terminal, and will kill him within a week. She takes a tearful leave of Ted and prepares to rush back to Sorensen's side. But then another earthquake in the Indian Ocean signals that the crack has not stopped, but has changed direction. Rampian asks Sorensen to be more specific, and when he hears where the new quake's epicenter was, he realizes that the crack has doubled back toward the Project's original bore hole–a course that might not have the dire consequences of cleaving the crust into two equal hemispheres. But the crack is now moving at twice its original speed.Rampian rushes back to South Africa to observe the crack's progress personally. He and Maggie soon realize that two cracks have opened in the crust, both converging on the bore hole. Rampian orders an evacuation of a nearby village–but can then only watch helplessly as an evacuation train heads directly for the crack, and is wrecked when the crack knocks over a trestle while the train is trying to cross it. Frustrated, Rampian orders another evacuation, and then he and Maggie return to the Project. There they find that Sorensen has ordered a full evacuation–but Sorensen has stayed below in the bunker and won't come up!Maggie and Rampian take the elevator down, to find Sorensen sitting alone at the consoles. He says that he will personally record what he believes will be the birth of a new moon. Of course, he really is suicidal, a fact he makes clear when he sends Rampian and Maggie out of the control room on separate errands–and then abruptly shuts the heavy door, locking himself in, and then shorts out the motors that move the door. Heartsick, Rampian and Maggie race for the elevator to take it topside–but then a boulder smashes into the elevator cab, stopping it about halfway up. Desperately the two climb through the trap door and up the elevator shaft, and then climb uphill away from the bore hole as fast as they can.Finally the two cracks meet at the bore hole, and a gigantic disk of crust lifts up into space, to join the Moon in orbit. After which the earthquake activity stops, and small animals come out of hiding as Rampian and Maggie comfort one another, happy to be alive in a world that can still sustain them.