Michael "Mike" Enslin is a cynical, skeptical author who is estranged from his wife Lily after the death of their daughter Katie. Mike writes niche books evaluating supernatural events in which he has no belief. After his latest book, he receives an anonymous postcard depicting The Dolphin, a hotel on Lexington Avenue in New York City bearing the message, "Don't enter 1408." Viewing this as a challenge, Mike arrives at The Dolphin and requests room 1408.
The hotel manager, Gerald Olin, attempts to discourage him. He explains to Mike that in the last 95 years, no one has lasted more than an hour inside of Room 1408; the latest count is 56 deaths, all suicides or "natural" deaths. The horrors began with a wealthy businessman named Kevin O'Malley, who inexplicably committed suicide in the room when the hotel opened. Olin even attempts to bribe him with a free room upgrade but Mike insists so preparations are reluctantly made.
Inside the room, Mike describes on his mini-cassette recorder the room's boring appearance and lack of supernatural behavior. The clock radio suddenly starts playing "We've Only Just Begun", a hit song by The Carpenters. At 8:07, the song plays again and the clock's digital display changes to a countdown starting from "60:00." Mike begins to see ghosts of the room's past victims, followed by flashbacks of Katie and his sick father. His attempts to leave are in vain: climbing through the air ducts prompts an attack from the corpse of O'Malley, and when he tries to escape to an adjacent room by climbing out the window and sidling along the outside ledge, he finds that the windows of the other rooms have disappeared.
Mike uses his laptop to contact Lily, pleading with her to call the police, but the sprinkler system short circuits his laptop. The room temperature drops to subzero when the laptop suddenly begins to work again. Lily tells him that the police have already entered 1408, but the room is empty. A doppelgänger of Mike appears in a video chat window and urges Lily to come to the hotel herself; it gives Mike a sly wink. The room shakes violently and Mike breaks a picture of a ship in a storm. Water pours from the broken picture, flooding the room. He surfaces on a beach and relives a surfing accident seen earlier in the film. His life continues from this point, and he reconciles with Lily.
Assuming his experience in 1408 was just a nightmare, Lily encourages him to write a book about it. When visiting the post office to send the manuscript to his publisher, he recognizes members of a construction crew as Dolphin Hotel staff. The employees then destroy the post office's walls, revealing that Mike is still trapped in the rubble of 1408. He sees Katie's ghost again and embraces her, and when the countdown ends, the room is suddenly restored to normal, and the clock radio resets itself to 60:00.
The "hotel operator" calls Mike. Mike asks why he hasn't been killed yet and she informs him that guests enjoy free will: he can relive the past hour over and over again, or use their "express checkout system." A hangman's noose appears and Mike has a vision of himself hanged, but he refuses to give in. Deciding to quit running, Mike improvises a Molotov cocktail and sets the room on fire. The hotel is evacuated. After smoking a cigarette, Mike breaks a window, causing a backdraft. He then lies down and laughs in victory upon destroying the room. Olin, in his office, praises Mike for his actions.
This one is the default ending of the theatrical release and its theatrical release DVD.
Mike survives, and he and Lily are back together, though Lily is skeptical of his experience. She finds a box of Mike's possessions that were rescued from 1408 and Mike takes the mini-cassette recorder from it, saying, "Sometimes you can't get rid of bad memories. You've just got to live with them." Lily realizes that Mike is ready to move on from his daughter's death. Suddenly, they hear Katie's voice on the recorder, confirming his account.
Director's Cut ending
Director Mikael Håfström said that the ending for 1408 was reshot because test audiences felt that the original ending was too much of a "downer." This first alternative ending was used in the theatrical release.
The original discarded ending had Mike dying from his fire injuries, but happy to see the room destroyed. During Mike's funeral, Olin approaches Lily and Mike's publisher Sam Farrell. He unsuccessfully attempts to give her a box of Mike's possessions, including the tape recorder. Olin claims that the room was successfully destroyed and that it will no longer hurt anyone else. He later listens to the recording in his car, and becomes upset when he hears Katie's voice on the tape. He sees a little girl walking on the cemetery grass behind the car, in a dress, calling out as if she is lost. He then sees Mike's burnt corpse in the backseat. Then he sees the same girl holding hands with her father as they walk away. Olin places the tape recorder back in the box and drives off. The final scene is of the gutted room, where an apparition of Mike looks out the window while smoking a cigarette. He hears his daughter calling his name, and disappears as he walks toward her. A door is heard closing and the scene fades.
This ending is the default ending on the Blu-ray release and two-disc collector's edition. Canadian networks Space and The Movie Network, and U.S. network FX broadcast this version of the film, but Space broadcast the theatrical ending on July 23, 2012. This ending is also used on the U.K. and Australian DVDs, and the U.S. iTunes, Netflix and Amazon Prime versions of the film.
Second alternate ending
Another ending uses elements from both the theatrical ending and the discarded original ending.
Mike dies from his fire injuries. Afterwards, Olin remarks "Well done, Enslin. Well done." Instead of the funeral scene, the sounds of a funeral are dubbed over shots of Los Angeles. Lily and Sam sort through Mike's effects. Sam returns to his New York office and discovers the manuscript that Mike wrote about Room 1408. As Sam reads the story, audio from Mike's experiences in the room is heard. In a final scene, Sam's office doors slam shut and Mike's father's voice says, "As I was, you are. As I am, you will be."
Third alternate ending
In a third ending, Mike survives and Lily moves to Los Angeles with him. When he plays the tape of Katie's voice from 1408, Lily hears it and looks shocked. Mike stares at Lily strangely. This is the ending shown on the Australian streaming service Presto and on Netflix in Japan. Netflix in the Netherlands used to show this ending, but later changed it to the ending used in the theatrical release.