Although David (Tobey Maguire) and his sister Jennifer (Reese Witherspoon) are twins, they lead dramatically different high school social lives. Jennifer is concerned mainly with her appearance, relationships and popularity, while David has few friends and cannot even drum up the courage to talk to a girl on whom he has a crush. He spends most of his spare time on the couch, watching television. Jennifer, on the other hand, is more assertive and at the beginning of the film makes a date with Mark Davis, one of the most popular boys in school.
Their mother (Jane Kaczmarek) leaves Jennifer and David alone at home while she heads out of town for a rendezvous with her boyfriend (who is later revealed to be nine years younger than she is). The twins begin to fight over the use of the downstairs TV; Jennifer wants to watch an MTV concert with Mark, while David needs the TV in order to watch a marathon of his favorite show, Pleasantville.
Pleasantville is a black-and-white '50s sitcom similar to Leave It To Beaver or Father Knows Best that centers around the idyllic Parker family—George (William H. Macy), his wife Betty (Joan Allen), and their two children, Bud and Mary Sue. David is an expert on every episode and wants to watch the marathon so he can win a trivia contest. During the fight between David and Jennifer, the remote control breaks and the TV cannot be turned on manually. A mysterious TV repairman (Don Knotts) shows up uninvited, and quizzes David on Pleasantville before giving him a strange-looking, futuristic remote control. The repairman leaves, and David and Jennifer promptly resume fighting. However, through some mechanism of the remote control, they are transported into the television, ending up in the Parkers' black and white Pleasantville living room. David tries to reason with the repairman (who communicates with him through the Parkers' TV set) but succeeds only in chasing him away. David and Jennifer must now pretend they are, respectively, Bud and Mary Sue Parker.
Breakfast in the Parker house is promptly served by stay-at-home mother Betty, and consists of generous servings of bacon, eggs, waffles, pancakes, ham, honey, sausage, and other fatty foods. Jennifer is disgusted at the thought of eating so much "animal fat." On the way to school, the pair watch as a group of firemen rescue a cat out of a tree, and Jennifer meets Skip (Paul Walker), the captain of the basketball team and her soon-to-be boyfriend. David tells her that they must stay “in character,” she must make small-talk with her three monochrome friends and not disrupt the lives of the Pleasantville citizens, who do not notice any physical differences between the old Bud and Mary Sue and David and Jennifer. In order to keep the plot in line, Mary Sue agrees to go on a date with Skip, although the two have very different ideas of what a date constitutes.
The date between Skip and Mary Sue turns out to be the first catalyst for change in the town. Skip has no knowledge of sex until Mary Sue introduces him to it. The plot of the traditional show is further thrown out of sequence when Bud’s boss Mr. Johnson (Jeff Daniels), who runs the soda shop, becomes dissatisfied with his boring, mundane life, confiding in Bud that the only time of the year during which he is happy is Christmas, due to the fact that he gets to paint something new every December 3rd for the Christmas mural in his shop's window. Bud initially attempts to convince him to carry on, saying that even if Mr. Johnson does not like his job, he should still do it anyway, but David soon realizes his error and gives Mr. Johnson an art book, encouraging his true passion.
Meanwhile, Skip tells the other boys about sex, and soon the teenagers begin to experiment, leading to a sort of sexual revolution. Betty is curious (leading to a sex talk between Betty and Mary Sue) and, knowing that her husband would never do any of the things Mary Sue describes, engages in masturbation while bathing. As she climaxes, a tree outside on the Parkers' lawn spontaneously combusts.
Bud, realizing the firemen have no other experience than fetching cats out of trees for neighbors, teaches them how to put out fires and is awarded a medal. He is thus noticed by a beautiful cheerleader named Margaret (Marley Shelton), who bakes him oatmeal cookies -- cookies she was supposed to bake for a boy named Whitey (David Tom). Bud’s act of heroism has inadvertently changed the storyline, but he seizes the moment and asks Margaret out for a date. When the TV repairman returns and berates him for altering the show so much, Bud turns off the TV, relinquishing his ability to go home in the process.
Pleasantville soon begins changing at a rapid pace. Double beds become available in stores, colored paints available to buy, students engage in sexual displays in public, and Pleasantville's beleaguered wives become tired of their household duties and begin to think, causing their husbands to reel in shock at their behavior. Meanwhile, things about the town which have changed from the original plotline begin to develop full and vibrant colors, rather than remaining black and white. The mayor, Big Bob (J.T Walsh) notices these changes and becomes concerned. He recruits George Parker, as a respected citizen, to the Pleasantville Chamber of Commerce to help normalize the town again, along with groups of other citizens who remain black and white. At this point, Betty has become "colored" as well and is afraid that George will hate her. Bud helps her to conceal the color with her old make-up, which is still black and white.
People in Pleasantville begin to explore hidden abilities and revel in their new freedoms. Mr. Johnson begins to paint, while Betty finds that housework no longer interests her. The basketball team loses their first game (previously, not only had they never lost, but they had never missed any shots), while students begin visiting the public library and reading books recommended by Mary Sue and Bud. Ironically, Mary Sue/Jennifer, who had never shown any interest in school, finds she likes reading so much that she rejects Skip in favor of a book by D. H. Lawrence, and finds her own color.
Gradually, more objects begin turning multicolor, including flowers and the faces of people who have experienced bursts of passion or change. The only people who remain unchanged are the town fathers, led by Mayor Big Bob who sees the changes as eating away at the moral values of Pleasantville. Certain youths, such as Skip and Whitey and their friends, also remain unaffected. They resolve to do something about their increasingly distant wives and disaffected youths. A town meeting is called. Betty falls in love with Mr. Johnson and leaves George for him, no longer wishing to hide her colored face.
Behavior similar to Nazism, as well as racial segregation and subsequent rioting similar to that of the African-American Civil Rights Movement, soon reach Pleasantville, touched off by a nude painting of Betty on the window of Mr. Johnson’s soda shop; the window is smashed with a park bench, and the soda shop is destroyed, piles of books are burned, and anyone who is "colored" is harassed in the streets. Bud earns his color by defending Betty from a gang of thugs led by Whitey.
He begins to grow from a quiet loner into a strong leader, advocating resistance to the new "Pleasantville Code of Conduct", a list of regulations preventing people from visiting the library and Lovers' Lane, playing loud music, or using paint colors other than black, white, or gray.
In protest against the mundane Pleasantville outlook, Bud and Mr. Johnson paint a colorful mural on a brick wall, depicting Martin Luther King, Jr.'s "I Have a Dream" speech, winged books rising from piles of burning literature, men and women dancing together to rock music, and other things relevant to the changes in their world. For this they are forced to spend the night in a jail cell. Bud is visited then by George, who wonders why Betty has changed, after he reveals he hasn't eaten in a very long time because he doesn't know how to cook. Bud simply replies that "people change," to which George wonders aloud if they couldn't just change back to the way things were.
Bud and Mr. Johnson are brought to trial in front of the entire town, with the monochrome citizens on the ground floor as witneses, segregated from the "colored" residents who are made to sit on a balcony as democratic voters.
George gains his color when, in the courtroom, he cries for the loss of his wife after Bud helps him realize the truth about what he actually misses (Betty herself, not the tasks she performs). Mr. Johnson is repentant and tries to haggle with the Mayor, but Bud speaks out, finally arousing enough anger and indignation in Big Bob that the Mayor himself becomes colored as well.
With this, the entire town becomes colored—and the people of Pleasantville are finally introduced to the rest of the world. Televisions at the television repair shop now display full-colored images of various scenic vistas around the world, such as the Pyramids at Giza and the Eiffel Tower, and Main Street, which had previously been a circuit that led back to its beginning again, now leads away to other streets, and ultimately to other towns and cities as well.
Jennifer chooses to stay behind in this alternate world, planning to go to university out of town as Mary Sue Parker. David returns using the remote control and finds his mother crying in the kitchen, distraught over her predictable, middle-aged life and her failed relationship with her junior lover. She complains to him that her life was not supposed to run this undesirable course.
David replies, saying, "It's not supposed to be anything."
The movie ends with a cut back to Jennifer/Mary Sue, reading a book to a sweetheart on the university steps, and with a shot of Betty and George, reunited; however, when Betty turns to look at her husband, it is Mr. Johnson who appears in his place.
William H. Macy
J. T. Walsh