As New York City detectives Sam Tyler and Maya Daniels lead police cars speeding along the freeway on their way to arrest a suspect in a series of serial killings, they discuss their relationship and whether Sam was drunk when he had told her it was time he met her folks He tells her they need to proceed with caution and he's crazy about her, but asks if they can talk about it later.
They meet with Captain Bellow and other officers at the suspect's apartment at the dilapidated Cataldo House. While the police are in the process of breaking down the suspect's door, Colin Raimes comes back with groceries. When he starts to run, Sam Tyler chases him down and arrests him for the abduction and murder of Laurie Stewart.
Back at the 125th Precinct, Sam, Maya and Bellow question Raimes about the crime. However, Raimes' lawyer produces a video that prove proves his client had been at a casino playing craps at the time of the murder, so he is released from custody.
After Raimes is released, Maya begins her own personal surveillance of Raimes and follows him as he walks through the East River Park, telling Sam on the phone that she is a cop first and foremost and needs to prove it to him. Meanwhile, Captain Bellow shows Sam photos of Lawrence Raimes, Colin's twin brother, who is a degenerate gambler. Fearing Maya is in dire danger because she isn't answering her cell phone, Sam, Captain Bellow, and a team race to the park, only to discover Maya's bloody jacket. Sam drives back to Colin's apartment to look for Maya. He gets out of his car and walks toward the dilapidated Cataldo House, but receives a call from dispatch on his police radio. When he returns to take the call, he is struck by a speeding car and is thrown to the ground. The car that hit him races away.
After regaining consciousness, a stunned Sam stands up and sees a sign advertising the construction of the Caltaldo House. Not realizing the change in decades, he walks over to an old Chevrolet Chevelle. As he peers inside the window, he sees an old 8-track tape playing the same song he had been listening to on his iPod when he was struck - David Bowie's "Life on Mars." A police officer walks up to the car and tells him he can't park his car there. He tries to explain that the Chevelle is not his car. He is dazed and confused. Then he looks up at the New York skyline and is stunned at the sight of the Twin Towers of the World Trade Center. He mutters, "no way."
Sam begins to walk the streets of the city trying to understand the strange sights and the old-fashioned clothes he's wearing. He returns to the 125th Precinct, where he is met by Detectives Chris Skelton and Ray Carling. Unsure if this is some sort of practical joke, Sam is nearly hysterical and asks what happened to his desk, his chair, and his computer. His question is answered by Skelton's naïve rejoinder, "like HAL in 2001?"
Sam is still confused about where he is and is told he must be the new detective who has been transferred from Hyde. When he asks what year it is, Lt. Gene Hunt enters the squad room, who introduces him to the "Chinese Year of the Fist" to his gut, and tells Sam it is 1973, this is his kingdom, and he is in charge.
Sam tries to call Maya's cell on the telephone, but is connected to a confused operator who accuses him of playing a joke. He hears the sounds of an operating room in the background, someone charging the paddles of a heart defibulator, and the voices of doctors explaining he's in a vegetative state. He begins to wonder if he is dead. He screams for the noises to stop. He cries out in pain, which earns him a trip to the precinct's nurse, Officer Annie "No-Nuts" Norris, for medical attention. Annie explains to him she is called "No-Nuts" because she's a member of the Bureau of Policewomen and isn't considered equal to her male counterparts. Sam tries to tell her he is from the year 2008, but she doesn't believe him and thinks he is delirious.
Hunt, Skelton and Carling tell Sam they've found a girl who had been missing for two days. Her blouse had been found in the East River Park, just like Maya's jacket was found in 2008. The girl, Susie Tripper, was found dead, and all the signs of her murder are similar to that of Laurie Stewart: She had been held for 30 hours before being strangled. When they visit the coroner's office, Sam is appalled at the lack of forensics in 1973, such as the two weeks required to get a fingerprint match. However, he finds something everyone else missed - synthetic fibers under the dead girl's fingernails that match the fibers from the Laurie Stewart case. Sam begins to believe he had been transported 35 years into the past to catch Colin Raimes. As he tries to pull his thoughts together, Gene Hunt asks Annie to take Sam to his new apartment to get some rest.
While in his apartment, Sam tries to convince Annie he is from the future and explains that Susie Tripper's killer could be the same person who had abducted his girlfriend in 2008. Annie tells Sam she believes he is suffering paranoid delusions. That night, Sam experiences more strange hallucinations and believes a man on television is talking to him, trying to wake him up from his coma. Is he in a coma? Before Sam can figure out what's happening, the television changes to an old-fashioned test pattern.
The following day at the 125th, Hunt, Skelton, Carling and Sam question Dora, a friend of Susie Tripper's. Dora isn't forthcoming with any information, and Sam is appalled at Lt. Hunt's rough tactics in questioning a witness. Later, Sam talks to Gene at a bar and attempts to convince him the killer is following a pattern. Hunt laughs him off.
Sam goes back to the precinct to review the case with the other detectives by using his 21st century technical expertise. He asks Annie, who has a psychology degree, to help him get into the mind of the killer. The detectives find the discussion about DNA tests boring and discount what Sam is trying to tell them, and Annie asks Sam never to put her on the line like that again.
Frustrated and unable to figure out whether all of this is happening in his mind, Sam and Annie leave the precinct and wanders into a record shop. As he looks at albums by artists that he considers collectors' items, his keen eye notices soundproofing material in the demo booth that is similar to the fiber he had found under Susie Tripper's fingernails.
Sam brings the fibers back to the precinct and explains to the other detectives that the fibers are from soundproofing material that the killer uses to muffle his victims' screams. As they listen to Sam's theory, Ray tells Sam that Dora is missing and her bloody jacket had been found at a subway station.
After reviewing the case file, Sam brings in Colin Raimes' grandmother for questioning again because she had made a complaint about noise in her apartment three months earlier. She refuses to speak to Sam, but Hunt is able to persuade her into providing an important detail. Her downstairs neighbor, Willy Kramer, used to play his records too loud. Since her complaint to the police, she hasn't heard a thing.
Sam and Lt. Hunt realize the importance of this news and go to Willy Kramer's apartment. After they break down the door, they find Dora tied up and surrounded by acoustic padding. Willy Kramer happens to be coming home with groceries. He is surprised at the presence of his visitors, and hits Gene and runs. Sam chases him through alleys, streets, and into an abandoned apartment. He thinks he has cornered Kramer, but after a scuffle, Kramer wrests Sam's gun from him and points it between his eyes.
Staring down the barrel of his own gun, Sam tries to reason with Kramer. When Kramer tells Sam, "It's the only way home," Sam believes his comment means if he dies in this time period, he can go back to 2008. Sam pleads with Kramer to kill him, and presses the barrel of the gun against his chest and begs Kramer to pull the trigger. The sounds of a heart monitor grow louder as Sam gets closer to death. Before Willy Kramer can pull the trigger, Gene Hunt clouts Kramer and disarms him, and Ray takes him into custody.
As Sam watches the officers take Kramer away, he see Kramer wave goodbye to a couple of twin boys, Colin and Lawrence Raimes. Sam watches Young Colin stare with admiration at Kramer, and he believes that Colin Raimes had grown up to be a killer based on his hero worship of Willy Kramer.
Later that night, Sam drives by Young Colin's house to speak to him about his idolization of Willy Kramer. Colin is upset at Sam for taking away his friend. Sam reaches for his revolver from the seat of the car as he speaks to Young Colin, and is about to shoot him until he hears Maya's voice on his radio telling him she is safe and wants him to come back home. Sam can't force himself to kill the boy and he doesn't know how to come home. As Young Colin asks him if he is okay, Sam gets a call from Ray to investigate an armed robbery. Believing he must still be in 1973 for another reason, he drives away to meet the other detectives at the new crime scene.