Mike Enslin writes books about haunted places, but he believes in none of it. He is bitter after the death of his young daughter Katie, and that loss also ended up breaking his marriage to Lily. While promoting a book in Hermosa Beach, California, he gets an anonymous postcard of The Dolphin, a hotel on Lexington Avenue in New York City. The message says "Don't enter 1408." Mike takes it as a challenge.

At the hotel, manager Gerald Olin tries everything to stop him. He offers a better room and free drinks, and he shows Mike files on the deaths. In 95 years, no one has lasted more than an hour in 1408, and the count stands at 56 deaths. Mike threatens legal action and gets the key.

At first the room looks normal, and Mike mocks it on his mini-cassette recorder. Then the clock radio starts playing "We've Only Just Begun" and begins counting down from 60:00. A window slams on his hand, hot water burns him, and ghosts of past victims appear, some of them jumping from the window. He also sees flashbacks of Katie and of his terminally ill father. He tries to escape along the outside ledge, but the other windows on the floor are gone.

Mike uses his laptop to reach Lily and beg for help. The sprinklers short out the laptop, and then the room freezes. A fake copy of Mike appears on the video chat and tells Lily to come to the hotel. When the room shakes, Mike crashes into a painting of a ship in a storm, and water floods out and drowns the room.

Mike wakes up after a surfing accident, as if the whole thing was a nightmare. In hospital he makes peace with Lily, who tells him to write a book about it. But at the post office, he recognizes the construction workers as Dolphin Hotel staff. They smash the walls, and he sees that he never left 1408. Katie's ghost comes to him and turns to ash in his arms. The countdown ends, the room resets, and the clock goes back to 60:00.

A woman's voice calls Mike on the room phone. She says guests have free will: he can relive the same hour again and again, or use the "express checkout." A noose appears. The room is not trying to kill Mike. It wants him to kill himself. Mike refuses. He makes a Molotov cocktail, sets the room on fire, and lies down laughing as it burns.

What happens next depends on which version you watch. There are four endings in total.

Theatrical ending: Firefighters rescue Mike, and he gets back together with Lily. She doubts his story until he fixes his damaged tape recorder and they both hear Katie's voice on it. So it was all real.

Director's cut: Mike dies in the fire. At his funeral, Olin tells Lily that Mike did not die in vain because the room is destroyed, but she refuses the box of his things. In his car, Olin plays the tape, hears Katie, and sees Mike's burned body in the back seat. The last scene shows Mike's ghost in the burned room, where he hears Katie calling him and walks away.

Other versions: In one, Mike dies and his publisher Sam Farrell finds the manuscript and hears the room on the tape. In another, Mike lives, and only he notices Katie's voice on the tape.

So what is the real ending? The theatrical one is what played in cinemas, and director Mikael Håfström said in 2024 that he finds it stronger and more emotional. Some critics prefer the darker director's cut. In Stephen King's short story, Mike has no daughter and survives by setting his own shirt on fire. He lives on with lasting damage and gives up writing.