The story revolves around a struggle to determine which of Satan's three sons will succeed their father (Harvey Keitel) as ruler of Hell. Adrian (Rhys Ifans) is the most devious, Cassius (Tom "Tiny" Lister, Jr.) is the cruelest, and Nicky (Adam Sandler) is their father's favorite. Adrian and Cassius claim that Nicky's mother is a goat and torment him by controlling his body with their minds. Nicky has had a speech impediment and a disfigured jaw since Cassius hit him in the face with a shovel.
Having been the Prince of Darkness for ten thousand years, the Devil assembles his sons to decide which of them will succeed him, but instead he keeps the throne for himself. Angered by this decision, Adrian and Cassius go to Earth to create a new Hell by possessing religious and political leaders in New York City. As they leave, they freeze the entrance to Hell, preventing more souls from entering and causing Satan's body to begin decomposing. To stop Adrian and Cassius, Satan sends Nicky to Earth with a silver flask that traps whoever drinks from it inside.
At first, Nicky has trouble staying alive on Earth. He is killed several times, landing in Hell and returning to New York each time. While learning how to eat and sleep, he meets a talking bulldog named Mr. Beefy (voice of Robert Smigel), rents an apartment with an actor named Todd (Allen Covert), and falls in love with a design student named Valerie (Patricia Arquette).
Nicky's first encounter with his brothers occurs when Adrian sees him with Valerie, takes mental control of his body, and makes him scare her away. Then Nicky sees Cassius on television, possessing the referee of a Globetrotters game. When he goes to the court and tricks Cassius into the flask, Satanist metalheads John (Jonathan Loughran) and Peter (Peter Dante) are so thrilled with his performance that they become his devoted fans. That evening, Nicky tries to apologize to Valerie. The meeting goes badly at first, but she accepts him after he explains who he is and why he is on Earth.
The next day, Adrian possesses the Chief of the NYPD (Michael McKean) and accuses Nicky of mass murder. Not knowing what to do, Nicky has Todd kill him so he can go back to Hell and ask his father for advice, but his father has trouble hearing because his ears have fallen off, and his assistants are in a panic because the midnight deadline to capture Adrian and Cassius is only hours away. Back on Earth again, Nicky and his friends devise a plan to capture Adrian in a subway station, but Adrian discovers their trick. In the ensuing fight, Adrian grabs Valerie and dives onto the track as a train approaches, but Nicky throws her out of the way, leaving himself and Adrian to be killed by the train.
Arriving in Hell just minutes before midnight, Adrian begins the process of taking over Hell by pushing what remains of his father aside and sitting on the throne, rising to Central Park, and starting a riotous party. Meanwhile, Nicky wakes up in Heaven as a reward for sacrificing himself and meets his mother (Reese Witherspoon), an angel who tells him he can defeat Adrian with the Inner Light he inherited from her. After she gives him a mysterious orb, he goes to Central Park. Adrian appears to win a pitched battle by locking Nicky in the flask and turning himself into a bat, but Nicky escapes from the flask. When he shatters the orb, Ozzy Osbourne appears, bites Adrian's head off, and spits it into the flask.
With his brothers captured, Nicky is ready to save his father. After he sins to make sure he goes to Hell, he and Valerie express their love for each other and she kills him. In Hell, Satan regains his body and recommends that Nicky go back to Earth. The film ends a year later, when Nicky and Valerie live in New York with their infant son. It is mentioned that John and Peter have died in a Zeppelin crash they bought and are now living in Hell in Nicky's old room.
Adam Sandler as Nicky
Rhys Ifans as Adrian
Tom 'Tiny' Lister, Jr. as Cassius
Liam Neeson as Dad (Satan)
Allen Covert as Todd
Patricia Arquette as Valerie Veran
Robert Smigel as the voice of Mr. Beefy
Rodney Dangerfield as Grandpa (Lucifer)
Jonathan Loughran as John
Peter Dante as Peter
Leah Lail as Christa
Jackie Sandler as Jenna
Dana Carvey as the Referee
Michael McKean as Chief of Police
Kevin Nealon as Gatekeeper
Reese Witherspoon as Holly (Nicky's Mother)
Blake Clark as Jimmy the Demon
Regis Philbin as Himself
Ozzy Osbourne as Himself
Bill Walton as Himself
Dan Marino as Himself
Henry Winkler as Himself
Rob Schneider as The Townie (Playing his character from The Waterboy)
Carl Weathers as Chubbs (Playing his character from Happy Gilmore)
Clint Howard as Nipples
Jon Lovitz as Peeper (Singing the same tune as was sung by his character in The Wedding Singer),
Fred Wolf and the Harlem Globetrotters
Radio Man as Himself
Quentin Tarantino Deacon
Ellen Cleghorne as Mom at Globetrotters game
Laura Harring as Mrs. Dunleavy