In an underground, totalitarian society in the future, sexuality has been outlawed and constant sedation of men and women is mandatory. But THX 1138 and his female roommate LUH 3417 have been cutting back on their medications and have fallen in love, so THX 1138 is imprisoned for drug evasion and misconduct of a sexual nature. With help from an inmate, THX 1138 breaks out to find LUH3417 who is pregnant. Because the funding for fugitive pursuits are quite limited, THX 1138 will survive if he can evade the robocops, but his vehicle keeps overheating and surveillance is everywhere.

In the early twenty-first century, computers manage human society, guiding people who are kept on a steady diet of sedatives that deaden the senses through their subterranean world. Assigned numbers instead of names, individuals are further stripped of their uniqueness by wearing the same white outfits and being shaved bald. As workers toil in nuclear laboratories where robotic police figures are constructed, soothing computer voices direct them through each movement, including the occasional catastrophic accident. One man, THX 1138, grows increasingly restless, unaware that his flat mate, female LUH 3417, has been rebelling against their emotionless existence by gradually reducing his normal drug intake. Confused by his mounting agitation, THX seeks comfort from a computerized confessional where he relates his anxiety to a televised image of a painting of bearded man while a computer voice drones sympathetic pre-recorded phrases. THX confides that his increasing inability to concentrate has led to minor problems at work that distress him, and admits to having difficulties with LUH. The computer bestows blessings of the state and the masses and urges THX to be grateful for his job, which allows him to be the perfect consumer. Back at his flat, THX seeks diversion watching holograms of erotic dancers, news, a comedy program and a discussion on spirituality. LUH anxiously listens as THX becomes fixated on a violent hologram of robotic police beating a man. The next day, THX returns to the confessional, which repeats the same recorded blessing. None of the surrounding hooded monks intervene when THX becomes violently ill in the booth. Later, THX manages to return home where LUH tends to him. The following morning, LUH acknowledges that she yearns for physical intimacy with THX, which is forbidden. When THX responds to her, LUH expresses fear that they are being observed, but THX assures her no one is watching them and the couple has sex. Afterward, LUH confesses having reduced THX's drug intake and cautions him that he cannot return to his regular dosage, especially if they have developed feelings for each other. Although LUH admits to her fear of being arrested for drug violations, she suggests that they run away and try to live together outside of the underground community. A few days later, LUH anxiously tells THX that she has had a work shift change and has been ordered to see her superior, SEN 5241. Unknown to the couple, SEN has monitored most of their recent exchanges over the last few days and, with his fellow monitors, knows of LUH's drug transgressions as well as the fact that the couple has had illegal intimate relations. That afternoon, SEN visits THX at work and discloses that LUH requested a transfer to new quarters. Doubtful, THX returns to the flat that evening and waits worriedly for LUH, but she does not appear. The next morning, SEN meets THX, who questions why he is in his work sector again. SEN blithely acknowledges that he can program his own shifts, then admits he transferred LUH and reminds THX that he needs a new roommate. Piqued by SEN's attitude, THX reports him for illegal program shifting. During work, however, THX is overcome by stress over LUH's absence and very nearly causes a major accident when he drops a nuclear rod. High-security monitors place a mind block on THX as his co-workers are ordered to leave the area and robotic police are dispatched to arrest him for drug evasion. THX is taken to a confinement area where he is beaten, but he refuses to make a statement. A trial is rapidly arranged and THX is brought up on charges of drug evasion and sexual perversion. Upon being found guilty he is sentenced to detention and conditioning, which consists of several rounds of electric torment, followed by numerous medical test procedures that lead to severe torture by unseen security monitors. After THX rests and recovers from the ordeal, LUH is allowed to visit him and reveals that she is pregnant. The couple is given permission to have sex, but when robot police arrive to escort THX away he resists and is knocked out and taken to a place of detention for defectives. Reviving, THX discovers SEN has been sentenced there, too, along with several others who have failed to conform. Quickly growing frustrated with the rambling monologues and bizarre behavior of the other detainees, THX decides to escape. SEN accompanies THX and the pair walk a great distance along a pure white expanse before spotting a man in the distance who waves at them. When the pair reach the man, he introduces himself as SRT, a hologram who has tired of his dull existence as an erotic dancer and is also attempting to escape. The men proceed and eventually come to a darkened area and a glowing ball before a doorway. Opening the door, the men are startled to find scores of people bustling along a wide walkway. THX and SRT struggle across the hordes of people, but SEN is swept along with them and separated from the others. Meanwhile, security has monitored THX and SEN's escape and orders an official search. As SEN boards a tram, THX and SRT wind their way through a room full of large jars holding fetuses and on to a vast circuit room, then find themselves in a darkened room with several bodies on gurneys. Just as THX inspects the bodies and realizes they have had all their organs removed, an attendant arrives to tag each corpse. When THX is almost tagged, he flees, followed by SRT. While SEN finds himself in a studio that broadcasts the confessionals, THX and SRT discover an uninhabited monitoring station. THX performs a computer search for LUH and learning that her name and number have been reassigned to a fetus, realizes that she is dead. The robotic police continue seeking THX while monitors keep track of the rising costs of the search. Meanwhile, SEN returns to a tram station and talks with several children who have just started their sedative regimen. THX and SRT find several jet cars parked near the monitoring station and THX breaks into one and drives away. SRT has difficulty starting his car and as the robotic police move in, he guns the car and slams into a cement post. THX races away through a long tunnel pursued by the robotic police on jet motorcycles. After one cyclist crashes against a construction zone, the other is finally ordered to cease the chase as THX's pursuit has gone over budget. Abandoning the car, THX climbs up through a long shaft and, opening a hatch, emerges into the outside world just as the sun rises.