The story revolves around a struggle to determine which of Satan's (Harvey Keitel) three sons will succeed their father as ruler of Hell. Adrian (Rhys Ifans) is the most intelligent, Cassius (Tom Lister Jr.) is the toughest, and Nicky (Adam Sandler) is their father's favorite, even though Nicky has had a speech impediment and a disfigured jaw since Cassius hit him in the face with a shovel. Furthermore, Adrian and Cassius enjoy tormenting Nicky by claiming that his mother was a goat and assault him by controlling his body with their minds.
Having been the "prince of darkness" for ten thousand years, Satan assembles his sons to decide which of them will succeed him, but instead he keeps the throne for himself because his sons are not yet ready to be his successor and tells them that they need to learn to keep the balance between good and evil. 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 via his body literally falling apart. 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) (a friend of Nicky's father), 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 (Dana Carvey) of a Harlem Globetrotters game. When he goes to the court and tricks Cassius into the flask, 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 using a rather badly edited scene from Scarface. 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; John and Peter inform the chief of the NYPD of Nicky's whereabouts which leads to Todd and Mr. Beefy being arrested. While waiting for Nicky, Adrian discovers their trick when he realizes that John is keeping cool whereas Peter isn't which leads him to realize that John is being possessed by Nicky. 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 dethrones his weakened father and takes over, rising into Central Park and starting a riotous party while all of the demons except Satan's assistant Jimmy (Blake Clark) join Adrian on Earth. Meanwhile, Nicky wakes up in Heaven as a reward for sacrificing himself and meets his mother Holly (Reese Witherspoon), an angel who tells him he can defeat Adrian with the "inner light" that he inherited from her. After she gives him a mysterious orb, he goes to Central Park. The demons discover Nicky and try to attack him, but Nicky creates gifts for them which makes them respect Nicky enough to join him and stop Adrian, but they all run away when Adrian kills one of them. Nicky and Adrian then fight, which results in both of them sucked into the flask where they have a three-way battle with Cassius. Adrian appears to win a pitched battle by locking Nicky in the flask and turning himself into a bat, but Nicky (with Valerie's help) escapes from the flask. When he shatters the orb, Ozzy Osbourne appears, bites Adrian's head off and spits it into the flask (that scene was inspired by an actual incident in which Ozzy bit the head off of a real bat while on stage).
With his brothers captured, Nicky is ready to save his father. After he sins to make sure he goes to Hell by setting bees on Henry Winkler, he and Valerie express their love for each other and she kills him with a boulder given to her by Ozzy. With the flow of souls restored in Hell, Satan regains his body and recommends that Nicky go back to live on Earth. As for punishment, Adrian and Cassius (still inside the flask) are shoved up Adolf Hitler's rectum. The film ends a year later, when Nicky and Valerie live in New York with their infant son, Zachariah. In in afterword in which what happens to the characters, it is mentioned that John and Peter have died in a plane crash and are now happily living in Hell in Nicky's old bedroom.
Output: What happens to someone who drinks from Nicky's flask?