Truman Burbank is the unsuspecting star of The Truman Show, a reality television program which is broadcast live around the clock and across the globe. His entire life has taken place within a giant arcological dome in Hollywood, fashioned to create the seaside town of Seahaven Island, and equipped with thousands of cameras to monitor all aspects of Truman's life. All of Seahaven's residents are actors, either acting out a script or repeating lines fed to them by the show's creator and executive producer, Christof, who seeks to capture Truman's real emotion and human behavior.
The producers discouraged Truman from wanting to travel beyond Seahaven by instilling him with aquaphobia through the "death" of his TV father in a boating accident, and by constantly broadcasting and printing messages of the dangers of travelling. Despite Christof's control, Truman manages to act in unexpected ways. During his college years, Truman was intended to fall in love with and marry co-student Meryl, but he fell in love with another actress, Sylvia. Sylvia managed to bring Truman out of the sight of cameras long enough to warn him that his reality is fake before she was taken away, with her "father" claiming they are travelling to Fiji. While Truman went on to marry Meryl, he continues to fantasize about Sylvia, using scraps from magazines to recreate her face in secret, and seeks travel to Fiji. Outside of the show, Sylvia has become part of a "Free Truman" campaign that demands the end of the show.
The film begins during the thirtieth year of the show. During the day, Truman notices strange occurrences that all seem centered on him (a falling spotlight, rain that only falls on him). Truman spots a disheveled man and recognizes him as his father, who had snuck back into the set, but actors quickly drag the man away. Despite efforts by Meryl and Truman's best friend Marlon to reassure Truman, Truman becomes even more suspicious about his life. One day, he takes Meryl by surprise by going on an impromptu road trip, but their way is blocked by apparent emergencies created by Christof. Meryl begins to break down from the stress, and during an argument with Truman, breaks character and is later taken off the show. Truman, depressed and confused, is consoled by Marlon, and Christof uses the opportunity to re-introduce Truman's father to the show properly, under the guise of having lost his memory after the boating accident, in the hope of bringing Truman back to some emotional stability and a controllable state.
Truman seems to recover, but the next day, the producers find Truman sleeping in his basement. Marlon is sent to check on Truman, only to find he has disappeared through a makeshift tunnel. Marlon breaks character, and Christof orders the first transmission cut in the show's history while a citywide search for Truman is launched, going to such extreme measures as causing the sun to rise hours ahead of schedule. Audiences around the world are drawn to this sudden change. Truman is found sailing out of Seahaven, having conquered his fear of water, and Christof resumes the broadcast as he sends a man-made lightning storm to try to capsize the boat. Network executives fear that Truman may die on live television, but Truman manages to persist. Realizing he cannot dissuade Truman any further, Christof ends the storm.
Truman continues to sail until his boat punctures the wall of the dome, to his surprise, and finds an exit door. Christof, speaking directly to Truman through a speaker system, tries to convince him to stay by stating that there is "no more truth" out in the real world and that by staying in his artificial world, he would have nothing to fear. Truman considers this, then states: "In case I don't see you... good afternoon, good evening, and good night," previously his unwitting catch-phrase, takes a bow, and leaves. Watching the show at home, Sylvia races to meet Truman, while Christof's supervisors end the show for the last time and the audience starts looking for something else to watch.