The story revolves around a struggle to determine which of Satan’s three sons will succeed their father as ruler of Hell. Adrian is the most devious, Cassius is the cruelest, and Nicky 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; he tells them that they are not ready yet, having decided to keep his throne. 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, rents an apartment with an actor named Todd, and falls in love with a design student named Valerie.
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 and Peter 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 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 Holly, an angel who tells him that 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. Adrian appears to win a 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.