The narrative centers on a fierce competition among Satan's three sons to determine who will inherit the throne of Hell. Satan, portrayed by Harvey Keitel, has three very distinct personalities vying for his attention: Adrian (played by Rhys Ifans), the most cunning of the trio; Cassius (portrayed by Tom Lister Jr.), known for his ruthless demeanor; and Nicky, the favored son, played by Adam Sandler. This rivalry takes a turn when Adrian and Cassius torment Nicky by claiming his mother is a goat and manipulating his actions with telepathic control. To add insult to injury, Nicky suffers from a speech impediment and a disfigured jaw due to a childhood incident with Cassius involving a shovel.
In a dramatic turn of events, Satan, having held the title of "Prince of Darkness" for ten thousand years, gathers his sons to choose a successor. However, he ultimately decides to retain his position, declaring they aren't ready yet. This infuriates Adrian and Cassius, who then make their way to Earth with a plan to establish their own dominion by taking control over influential leaders in New York City. Their actions lead to a catastrophic freezing of Hell's entrance, halting the influx of new souls and causing Satan’s physical form to decay.
To counter his sons, Satan sends Nicky to Earth with a mystical silver flask that traps anyone who drinks from it. Nicky finds himself at a loss for survival, dying multiple times and making brief returns to Hell as he learns to navigate life on Earth. He encounters a talking bulldog named Mr. Beefy, voiced by Robert Smigel, rents a place with an aspiring actor named Todd (Allen Covert), and ultimately falls for a design student named Valerie, played by Patricia Arquette.
Their relationship hits a snag when Adrian uses his powers to scare Valerie away from Nicky. Adrian also possesses a referee during a Harlem Globetrotters game, showcasing his villainy to the world. Nicky often confounds expectations, especially when he captures Cassius using the flask, much to the delight of two enthusiastic Satanist metal-heads, John (Jonathan Loughran) and Peter (Peter Dante), who become devoted followers after witnessing his exploits.
The action escalates further when Adrian manipulates the Chief of Police into accusing Nicky of heinous crimes, using edited footage to frame him. In a desperate move, Nicky has Todd kill him to seek guidance from Satan, only to find his father in a state of panic and agony. The plot thickens as Nicky and his allies devise a plan, which ultimately leads to a showdown in a subway station with Adrian, who seizes Valerie as a hostage in a climactic struggle.
In a last-ditch effort, Nicky sacrifices himself to save Valerie, landing him in Heaven where he reunites with his mother, Holly, played by Reese Witherspoon. She gifts him the "Inner Light," which is essential for defeating Adrian. Armed with newfound power, Nicky ventures back to confront his brothers.
In the ensuing chaos, Nicky manages to rally the demons by converting his approach, creating gifts to earn their respect. A fierce battle unfolds between Nicky, Adrian, and Cassius within the confines of the flask. Just when it seems Adrian has the upper hand, Nicky's cleverness prevails as he escapes and brings in the legendary Ozzy Osbourne to aid in the conflict.
With Adrian and Cassius subdued, Nicky is prepared to rescue his father, leading to a heartfelt reunion. He then chooses to embrace his own mistakes, resulting in a new love with Valerie and a return to the world of living.
The story concludes a year later with Nicky and Valerie settling down in New York, where they share the joy of parenthood with their infant son, Zachariah. In a humorous twist, it's revealed that John and Peter have perished in a plane accident but are now enjoying their afterlife in Nicky's former room in Hell.
