Sometime in the near future a high-tech, highly-realistic adult amusement park called Delos features three themed "worlds" — West World (the American Old West), Medieval World (medieval Europe), and Roman World (pre-Christian Rome). The resort's three "worlds" are populated with lifelike androids that are practically indistinguishable from human beings, each programmed in character for their assigned historical environment. For $1,000 per day, guests may indulge in any adventure with the android population of the park, including sexual encounters and even a fight to the death, depending on the android model. Delos' tagline in its advertising promises "Have we got a vacation for you!"
Peter Martin (Benjamin), a first-timer, and his friend John Blane (Brolin), who has visited previously, visit West World. One of the attractions in West World is the Gunslinger (Brynner), a robot programmed to instigate gunfights. The firearms issued to the park guests have temperature sensors that prevent them from shooting humans or anything else living, but allow them to "kill" the "cold blooded" androids. The Gunslinger's programming allows guests to outdraw it and "kill" it, always returning the next day for a new duel.
The technicians running Delos notice problems beginning to spread like an infection among the androids: the robots in Roman World and Medieval World begin experiencing an increasing number of breakdowns and systemic failures, which are said to have spread to West World. When one technician scoffs at the "analogy of an infectious disease," he is told by a resort scientist, "We aren't dealing with ordinary machines here. These are highly complicated pieces of equipment, almost as complicated as living organisms. In some cases, they've been designed by other computers. We don't know exactly how they work."
The malfunctions become less peripheral and more central when an android rattlesnake succeeds in injuring Blane in West World, and, against its programming, an android refuses a guest's sexual advances in Medieval World. The failures escalate until Medieval World's robotic Black Knight kills a guest in a swordfight. The resort's supervisors, in increasing desperation, try to regain control by shutting down power to the entire park, but this traps them in the control rooms, unable to turn the power back on while the robots run amok on reserve power.
Martin and Blane, passed out drunk after a bar-room brawl, wake up in West World's bordello, unaware of the breakdown. When the Gunslinger challenges the two men to a showdown, Blane treats the confrontation as a typical amusement until the robot shoots and actually hits him, eventually killing him. Martin runs for his life as the robot implacably follows him.
Martin flees to the other areas of the park, but finds only dead guests, damaged robots and a panicked technician. He climbs down through a manhole in Medieval World to the underground control area and discovers that the resort's technicians suffocated when the ventilation system shut down. The Gunslinger stalks Martin through the underground corridors. Ambushing it, Martin throws acid into its face and sets fire to it with a torch. He tries to rescue a woman chained up in a dungeon, but she turns out to be an android. The burned hulk of the Gunslinger attacks him one last time before succumbing to its damage. Martin, apparently the sole human survivor, sits down on the castle steps in a state of near-exhaustion and shock, as the irony of Delos' slogan resonates: "Have we got a vacation for you!"