As told through interviews and footage through the film, 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, Florida, and equipped with thousands of cameras to monitor all aspects of Truman's life. All of Seahaven's residents are actors. Creator and executive producer Christof wants to capture Truman's real emotion and human behavior.
The producers have 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 set to fall in love and marry co-student Meryl, but 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 and off-set, with her "father" claiming they are traveling 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 all seem centered on him (a falling spotlight, rain that only falls on himself). Truman spots a disheveled man and recognizes him as his father, who had snuck back into the set, but the 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 proper, hoping to bring Truman back to some emotional stability.
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 of the show's history while a city-wide search for Truman is launched. 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", takes a bow, and leaves. Sylvia races to go meet Truman while audiences celebrate. Christof's supervisors end the show for the last time, much to Christof's dismay, as audiences now start looking for something else to watch.

Question: What does Marlon discover in Truman's basement?
Answer: A hole that Truman used to escape into the town of Seahaven, without the cameras capturing his movements.
Alternative answer: The tunnel he escaped from.