At Game 6 of the 1977 World Series, Joe "Coop" Cooper, catches the ball from Reggie Jackson's third home run and proclaims to his best friend, Doug Remer, that "one day, I'm gonna be a big sports star."
16 years later in 1993, Coop (Trey Parker) and Remer (Matt Stone) are unemployed and about to lose their mortgage on their house. They arrive uninvited at a party hosted by a former high school classmate of theirs. After finding out that their classmates have grown-up and moved on with their lives, Coop and Remer get kicked out of the house and find themselves outside on the driveway basketball court.
There, they are dared by two other classmates to a game. The two see that their opponents are very good at basketball, so they say they will only play a new game they picked up "in the hood". Clearly making this new game up as they go, Coop originally proposes Horse, but changes it to basketball with baseball rules. During the newcomer's first throw, Coop "psyches" him out to make him miss; a rule not explained previously. A "psyche out" can be anything said or done that makes the offense lose their concentration and miss their shot. They then proceed to continue playing their new game, "BASEketball" and form their own league, recruiting a third member Squeak (Dian Bachar).
Six months later, people come from miles around and crowd their neighborhood to watch them play the game they created. Ted Denslow (Ernest Borgnine) shows up to propose creation of the National BASEketball League (NBL). Five years after creation of the league, the NBL is in full swing with stadiums, teams, fans, and a major championship (the Denslow Cup).
During the championship Denslow, who is the owner of the Milwaukee Beers for whom Coop and Remer both play, chokes on a hot dog and dies. After the game, Jenna Reed (Yasmine Bleeth), head of the Dream Come True Foundation, introduces herself to Coop and Remer as they leave the stadium. The reading of Denslow's will reveals that Coop becomes owner of the Beers instead of his widow Yvette Denslow (Jenny McCarthy) and will retain ownership if he wins the next Denslow Cup, otherwise ownership goes to Yvette.
Denslow's rival Baxter Cain (Robert Vaughn), who owns the Dallas Felons, wants to change the league rules that prohibit player transfers, teams moving to other cities, and corporate sponsorships to increase profits. Yvette would have been willing to comply had she been given ownership of the team, but Coop refuses to accept any of the rule changes, so Cain and Yvette come together to form a plan to make sure Yvette gets ownership of the team.
In a private conversation at Cain's office, Cain tells Remer that Coop has said no to Cain's rule-change plans without talking to the other members of the Beers. Remer then goes to the Beers behind Coop's back and tells the team what he learned from Cain. After Remer and the other members of the Beers confront him, Coop agrees to split ownership with Remer and the team. The team continues to agree that the rules should not be changed. Coop also seemingly enters into a relationship with Jenna, despite Remer's attempts to get between them.
Cain cuts the funds to Jenna's foundation, forcing Coop and Remer to ask Cain for help. Cain suggests creating a clothing line and sending the proceeds to her foundation. Coop is entirely against it, but Remer, as part team owner, immediately agrees, and becomes obsessed with his newfound fame that he alienates his friendship with Coop. After winning the league semifinals, Cain informs Coop and Remer through photos that their clothing line has been produced through child labor in Calcutta. If the public learns about it, the team and Jenna's foundation will be ruined and Cain threatens to show the photos to the public unless Coop and Remer lose or skip the championship game. Jenna learns of this and breaks it off with Coop. Coop and Remer finally have a falling out, after which Coop decides to go to Calcutta to resolve the situation.
Coop trades all the child workers in the factory for adult ones and makes it back just as the fifth annual Denslow cup begins. The Beers start with an abysmal performance, failing to make one hit in six innings. At the seventh-inning stretch, the Beers are down 16–0. After a moving speech from Squeak, Coop and Remer become friends again and Yvette breaks off her alliance with Cain. Coop, Remer, and Squeak finally get back into the game and start scoring.
In the bottom-half, Remer is on second, Squeak is on third, and Coop is up when his custom-made BASEketball (La-Z-Boy) pops. One of the boys from Jenna's foundation Joey (Trevor Einhorn) brings Coop a new custom-made BASEketball made from a Barcalounger for Coop to use. Coop misses, but successfully completes the conversion for the win and the Denslow Cup. Coop and Jenna reunite while Remer hooks up with Yvette.
Trey Parker as Joe "Coop" Cooper
Matt Stone as Doug Reemer
Dian Bachar as Kenny "Squeak" Scolari
Yasmine Bleeth as Jenna Reed
Jenny McCarthy as Yvette Denslow
Ernest Borgnine as Ted Denslow
Robert Vaughn as Baxter Cain
Trever Einhorn as Joey Thomas
The film did modestly at the box office but now enjoys a cult following.