A narrator named Ramirez tells a story his mother used to tell him. Sometimes, the Devil walks the earth in human form. He gathers guilty people in one place, hides among them, and tortures them before taking their souls to Hell. The story always begins with a suicide. Moments later, a man jumps from the 35th floor of a Philadelphia office tower, landing on a truck that silently rolls away.

Detective Bowden, a recovering alcoholic, is assigned to the suicide. Five years earlier, a hit-and-run driver killed his wife and young son and was never caught. All the police found was a note that said "I'm so sorry." At the same time, an elevator in the tower gets stuck with five strangers inside: Tony, a quiet mechanic and Marine veteran; Vince, a loud mattress salesman; Ben, a temp security guard with a violent past; Sarah, a young woman married to a rich man; and a polite old woman. The security team can watch them on camera and talk to them through a speaker, but they cannot hear anything back.

The power starts failing. Each time the lights go out, something evil happens. Ramirez sees a screaming, black-tear-streaked face flash on the elevator monitor and becomes convinced this is his mother's Devil's Meeting. Lustig, his boss, thinks he is crazy. The repairman Dwight falls to his death on top of the elevator car, and Lustig is electrocuted in the basement while trying to restore power. Inside the car, panic grows. Vince is thrown into the mirror and his jugular is sliced by a shard of glass. The old woman is found hanging from a light cable - dead, or so it seems. Ben's neck is snapped in the dark. Sarah's throat is cut.

Outside, Bowden uses the sign-in sheet to identify everyone - except the mechanic, who never signed in. A young woman arrives at the building and reveals he is her fiance, Anthony "Tony" Janekowski. And "Jane Kowski," the old woman's name, was never real at all. The devil borrowed the name from a stolen wallet.

When Tony says he deserves to be punished, the old woman - alive again - reveals herself as the Devil and screams at him. He grabs the dead repairman's radio and confesses: he was the hit-and-run driver. He killed a mother and her son five years ago on Bethlehem Pike, and he left the "I'm so sorry" note. Detective Bowden hears every word over the speaker. The confession robs the Devil of his prize. Furious, he vanishes, and the elevator starts moving again. Tony is the only passenger left alive.

As the bodies are wheeled away, Bowden takes Tony into custody. In the car, he tells Tony the truth: it was his wife and son Tony killed. Tony begs for the punishment he was promised. But Bowden, who once planned exactly what he would do if he ever met the driver, does something else. He forgives him. Ramirez closes the film with his mother's comfort: "Don't worry. If the Devil is real, then God must be real too."

The ending works on two levels. First, the mystery: the old woman was the Devil the entire time. She killed the passengers one by one while pretending to be one of them, and she even faked her own hanging death - the same trick as the killer in Agatha Christie's And Then There Were None, which Shyamalan openly called his inspiration. Her stolen name "Jane Kowski" is a hint that she was never a real person at all.

Second, the theme: repentance defeats the Devil. Every passenger trapped in the elevator was guilty of something real. Vince ran a Ponzi scheme that ruined people. Sarah was a con artist planning to rob her husband. Ben beat a man into a coma. Only Tony carried a sin he could not live with, and only Tony confessed. The moment he admits what he did over the radio, the Devil loses all power over him. The story's message is simple: guilt and lies are what feed evil, and confession and forgiveness are what break it.

Bowden's choice is the real twist of the film. He spent five years drinking over his family's death, dreaming of revenge on the driver. When he finally has the man in his car, he forgives him instead. That act answers the film's opening question - is there a Devil? Yes. But if the Devil is real, God must be real too, and forgiveness is stronger than vengeance. The final scenes, with Tony alive and taken to face earthly justice, complete the idea that justice and mercy can exist together.