Truman Burbank is the unsuspecting star of The Truman Show, a reality television program in which his entire life, since before birth, is filmed by thousands of hidden cameras, 24 hours a day, 7 days a week, and is broadcast live around the world. The show's creator and executive producer Christof is able to capture Truman's real emotion and human behavior when put in certain situations. Truman's hometown of Seahaven is a complete set built under a giant arcological dome in the Los Angeles area. Truman's family and friends are all played by actors, allowing Christof to control every aspect of Truman's life.
During the 30th year of the show, Truman notices certain aspects of his near-perfect world that seem out of place. A theatrical light (labeled Sirius) falls from the artificial morning sky, nearly hitting him (quickly passed off by local radio as an aircraft in trouble that began "shedding parts") and Truman's car radio picks up a conversation between the show's crew tracking his movements. Truman also becomes aware of more subtle abnormalities within his regular day-to-day life, such as the way in which the same people appear in the same places at certain times each day and Meryl's tendency to blatantly advertise the various products she buys.
To prevent Truman from discovering his false reality, Christof has invented means of dissuading his sense of exploration, the most drastic of which was "killing" his father in a storm while on a fishing trip to instill in him a fear of the water. Christof also floods the channels with news reports and commercials about the dangers of traveling, and television shows about how good it is to stay at home. The actor that played Truman's deceased father sneaks back onto the set as a homeless man, but is whisked away as soon as Truman notices him. Despite Christof's control, Truman has managed to behave in unexpected ways, in particular by falling in love with an extra, Sylvia, known to Truman as Lauren, instead of Meryl, the character intended to be his wife. Though Sylvia is quickly removed from the set and Truman marries Meryl, he continues to secretly pine for 'Lauren'. Sylvia becomes part of a "Free Truman" campaign that fights to free him from the show and to reveal his identity.
Despite the best efforts of his family and his best friend Marlon to reassure him, all these events cause Truman to start wondering about his life, realizing how the world seems to revolve and shape around him, with complete strangers to him knowing his name. Truman attempts to leave Seahaven but is blocked by emergency situations created by Christof to prevent Truman from leaving, further aggravating him. Meryl grows increasingly stressed by the pressure of perpetuating the deception, and their marriage unravels in the face of Truman's increasing skepticism and attendant hostility towards her. This causes her to snap and partially reveal Truman's identity and life in front of Marlon as well, making Truman confused.
As a result, Meryl is pulled off the show, and Christof officially brings back Truman's father, hoping his presence will keep Truman from trying to leave. However, he only provides a temporary respite: Truman soon becomes isolated and begins staying alone in his basement. One night, Truman fools the cameras and escapes the basement undetected via a secret tunnel, forcing Christof to temporarily suspend broadcasting of the show for the first time in its history. This causes a surge in viewership, with many viewers, including Sylvia, cheering on Truman's escape attempt.
Christof orders every actor and crew member to search the town, even breaking the town's daylight cycle to help in the search. They find that Truman has overcome his fear of the water and has sailed away from the town in a small boat named Santa Maria. After restoring the broadcast, Christof orders the show's crew to create a large storm to try to capsize the boat, prompting a heated debate with his superiors over the morality and legality of murdering a human being in front of a live audience. Truman almost drowns, but his determination eventually leads Christof to terminate the storm.
As Truman recovers, the boat reaches the edge of the dome, its bow piercing through the dome's painted sky. An awe-struck Truman then discovers a flight of stairs nearby, leading to a door marked "EXIT". As he contemplates leaving his world, Christof finally turns to his last resort by speaking directly to Truman via a powerful sound system, trying to persuade him to stay and arguing that there is no more truth in the real world than there is in his own, artificial world. Truman, after a moment's thought, delivers his catchphrase ("In case I don't see you... good afternoon, good evening, and good night"), bows to his audience and steps through the door and into the real world.
The assembled television viewers excitedly celebrate Truman's escape, and Sylvia quickly leaves her L.A. apartment and departs to the edges of Seaheaven so she can meet up with him. A network executive orders the crew to cease transmission of the show, as viewers look for something else to watch.