In this very unusual comedy, the devil meets an angel at a certain heaven-hell mixer; both drunk, they have a rendez-vois that produces three kids who somehow end up living in hell with the devil. The movie starts with a picture of hell - a guy falls off a tree spying on a woman taking off her clothes in her own house, ends up in hell. The three kids - Adrian, Cassius, and Nicky - are each very different. Cassius and Adrian live up to their rep of being the devil's children, while Nicky isn't 'into evil,' and is shown as a 'papa's boy.' The movie starts with Cassius and Adrian conspiring to take over Earth by leaving hell and messing up the balance between good and evil, thus taking power over everyone in the process. Their departure 'freezes the fire' through which souls enter hell, however, and the devil begins to fall apart, since no new souls can enter. He sets his only son left, Nicky, on a quest to go to Earth, find his brothers, and trap them in a silver flask, since they have to return to hell at the same time.
Nicky hesitantly takes his father's orders and, with the help of a talking dog his father asked to help, settles into an apartment with a wannabe actor. He's soon found by two devil-worshiping guys who help him retrieve his flask as it's stolen by a petty thief. The guys befriend him and stand by his side against his brothers. Nicky also meets a girl he gets infatuated with, though he doesn't tell her the truth about him yet. She helps get his flask back from the thief, and they... hit it off.
He's able to track down one of his brothers - Cassius - as he possesses the referee at a Globetrotters game. Adrian turns out to be far more witty and doesn't fall for a trap Nicky and all his new friends set up for him at a subway station. Meanwhile, the city of New York becomes unbalanced after the work the two brothers do to destroy it. Adrian summons hundreds of 'followers' who gather to have him lead them on in the end. However, he tells them that at midnight the whole city will turn into hell and they'll all die. Right before that, though, Nicky's brought to heaven when he dies saving his girlfriend's life. He meets his mom, an angel, who tells him the whole story and gives him a bright light in shape of a ball; she says God gave it to her and Nicky would know when to use it. He then emerges at the scene where a crowd gathers for his brother Adrian. He fights him, but Adrian wins and turns into a bat. Nicky then takes out the glowing white ball. It turns into Ozzy Ozbourne, who bites Adrian's head off and spits it into the flask, where the other brother already sits. So, Nicky saves the day, and hell, and his father.
At the end, he gets married to the girl, and their kid is, well, a quarter devil. The devil-worshipers take the money promised them by Adrian for giving out Nicky's location earlier, and by Lynyrd Skyynard's old airplane. They crash it and later take Nicky's place in hell.