An international consortium of scientists, operating as Project Inner Space in Tanganyika, Africa, is trying to tap into the Earth's geothermal energy by drilling a very deep hole down to the planet's core. The scientists are foiled by an extremely dense layer of material. 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 which 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 along a fault line, causing earthquakes and tsunamis along its path. Rampion warns a committee of world leaders that the crack is capable of extending beyond the fault, and that if it were to encircle the Earth, causing the land masses to split, the oceans would be sucked in, generating steam at high enough of a pressure to rip the Earth apart. Sorenson meanwhile discovers that there was a huge reservoir 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, lowered into the magma chamber of an island volcano in the path of the crack, is used in the hope of stopping the crack, but it only changes the crack's direction. Eventually, the crack approaches 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 the test site in time to observe the fiery birth of a second moon. Its release stops the crack, and the Earth survives.

In the near future, the United States has lost the war on drugs. Substance D, a powerful drug that causes bizarre hallucinations, has swept the country. Approximately 20 percent of the population is addicted. The government has developed an invasive, high-tech surveillance system and a network of undercover officers and informants. Bob Arctor is an undercover agent, assigned to immerse himself in the drug's underworld and infiltrate the supply chain. Arctor has a vision of being in his house with a wife and two children in Anaheim, California; today he has two drug-addicted, layabout housemates: Luckman and Barris. The three spend time taking D and having complex, possibly paranoiac examinations of their experiences. At the police station, Arctor maintains privacy by wearing a "scramble suit" that constantly changes every aspect of his appearance and voice; he is known only by the code name "Fred". Arctor's senior officer, "Hank", and all undercover officers wear scramble suits, protecting their identities even from each other. Since going undercover, Arctor has become addicted to Substance D, which he buys from Donna, whom Arctor hopes to purchase large enough quantities of D from so that she is forced to introduce him to her supplier. They have a tense, at times caring romantic relationship but she rebuffs his physical advances. At work, "Hank" orders "Fred" to increase surveillance on Arctor and his associates. Arctor's house is at the center of his investigation, since this is where Donna and the other addicts spend time. Arctor is inexpertly negotiating a double life and his use of D is damaging his brain. Barris is informing on Arctor to "Hank", arguing that Arctor is a terrorist, whilst also angling to be hired as a cop. Barris unknowingly conveys this information in the presence of Arctor, who is hidden behind his scramble suit. "Hank" reveals to "Fred" that he has long known that he is Arctor, who seems surprised, and repeats his name in a disoriented, unfamiliar tone. "Hank" informs him that the real purpose of the surveillance is to catch Barris and that the police were deliberately increasing Barris' paranoia until he attempted to cover his tracks. "Hank" reprimands Arctor for becoming addicted to Substance D and warns him that he will be disciplined. "Hank" explains how seriously brain-damaged Arctor has become from D and "Hank" "phones" Donna, asking her to come pick up Arctor and take him to New-Path, a corporation that runs rehabilitation clinics. "Hank" leaves and in private removes his scramble suit, revealing Donna. At the New-Path clinic, Arctor and other D addicts show serious cognitive deficiencies. "Donna", now known as Audrey, meets with Mike, a fellow police officer. They discuss how New-Path is secretly responsible for the manufacture and distribution of Substance D. Audrey expresses her growing ethical aversion to their police work, in which they deliberately recruited Arctor—without his knowledge—to become addicted to D; his health sacrificed so that he might eventually enter a New-Path rehabilitation center unnoticed as an addict and collect incriminating evidence of New-Path's D farms. Audrey and Mike debate whether Arctor's mind will recover enough to grasp the situation. New-Path sends Arctor to a labor camp at an isolated farm, where he mindlessly repeats what others tell him. Tending to corn crops, Arctor discovers hidden rows of the blue flowers that produce D. He secretly hides one flower in his boot, to bring to his "friends" at his next holiday from the farm.