The film opens with an animated sequence featuring Fat Albert (Kenan Thompson) performing the original series' theme song "Gonna Have A Good Time".
In real life, Doris (Kyla Pratt) is a sad teenager still recovering from the death of her grandfather, ignoring her foster sister, Lauri's (Dania Ramirez), efforts to bring her a social life. Upon learning that her parents will be away for the weekend, Doris sadly lets a tear hit her TV remote, as the Fat Albert and the Cosby Kids is on. The tear opens up the TV world to the real world and Fat Albert and the gang (Keith Robinson, Shedrack Anderson III, Jermaine Williams, Alphonso McAuley, Aaron Frazier and Marques Houston) (minus Bill's little brother Russell (Jeremy Suarez) go to help her. Doris insists that she is fine, but the gang can't leave yet - their show is over and they have to wait until the next day's show to go back. They follow Doris to school the next day and are amazed by the new technology like laptops and the internet. Meanwhile, Russell remains in the animated world trying to defend the junkyard from bullies who want to take over since Fat Albert is away.
Fat Albert also notices Lauri and falls in love with her. In another attempt to help Doris, the gang persuades some cheerleaders to invite them all to an outdoors party. With some reluctance, Doris agrees to attend. While they're there, Lauri dances with Fat Albert. Lauri's annoying ex-boyfriend Reggie (Omarion), desperately attempts to make her jealous by dancing with Doris. When Lauri doesn't notice him, he tries to kiss Doris. Doris is offended and causes a scene. Doris runs from the party and Fat Albert warns the boy to stay away from Doris. The next day, Doris goes to school, but asks the gang to go to the park instead of following her. Weird Harold, normally very clumsy, joins in a basketball game and is able to play perfectly. Mushmouth, who can't talk normally, is taught how to speak by a little girl. Dumb Donald goes to the library and is able to read and remove his pink face-covering hat.
When Doris gets them and takes them back to her TV, Bucky, Dumb Donald (who's eyes float in the TV without the hat on), and Weird Harold jump into the TV but the show ends before the others can enter. Fat Albert and Bill have an argument in private about going back (Fat Albert wants to stay in the real world with Lauri). Meanwhile, Rudy has fallen in love with Doris and asks her if he was a real person would she ever date him and she said yes. Searching for guidance, Fat Albert literally meets his maker, Mr. Cosby (Bill Cosby as himself) and tells him of the dilemma. Mr. Cosby tells him that his character is based on Doris's grandfather, which explains Doris's confusion over why Albert seems so familiar to her. Mr. Cosby then tells Fat Albert that he has to return to the TV, or he will turn into celluloid dust. Devastated, Fat Albert tries to tell Lauri that he has to leave, but she doesn't believe he's from the TV and thinks that he is just being insensitive.
The next day, Bill, Mushmouth, and Rudy jump back into the TV. Fat Albert waits and goes to a track meet that Doris and Lauri have, then takes them home and jumps into the TV (this convinces Lauri that Fat Albert wasn't lying to her).
At the end of the movie, Mr. Cosby and his old friends (who the characters in the show were based on) stand in front of their old friend Albert Robertson's grave. Doris is also there. Then the group of men have a little race, showing no matter how old they are, they are still kids at heart, the same kids from the TV show that they helped Bill Cosby inspire.
Before the end credits start, Fat Albert is trying to leap out of the movie screen but his pals are pulling him back. He points out that people in the audience need help and he even tells someone in the back of the theater that he/she needs to come back and finish watching the credits. The boys pull Fat Albert back inside and the end credits resume beginning with the a blend of both the animated characters and the live-action characters singing the Fat Albert Theme Song
- Chip, Dale, Miss Piggy, Huey, Dewie, Louie, Ttark, Tommy Pickles, all characters are heroes will guest star in this movie.