El detective Sam Tyler, en la investigación de un asesino en serie, se queda inconsciente tras un atropello de coche y despierta en 1973, donde es un detective recién transferido a inspector en la misma comisaría. Al principio se niega a aceptar su situación, le resulta difícil adaptarse a las actitudes y la tecnología de la época. La investigación de un asesinato en 1973, evidencia una posible conexión con los acontecimientos de la actualidad.

Sam investigates a series of fatal robberies at cash-checking stores while evidence points to someone in the squad being involved.

The murder of a returning Vietnam veteran has Lieutenant Hunt outraged, as he demands a quick resolution to what quickly becomes a complicated and controversial case. As Sam and the squad's team of detectives explore the circumstances surrounding the homicide, Tyler is sent down a mysterious path that may provide clues about his own family's past and why he is back in 1973

Sam finds his mother, Rose, in debt to a local mobster known for killing women that cause him trouble. Sam finds out the cops of the 125 are in cahoots with the mobster and ignore his illegal activities for tips on other mobsters.

Racial tensions run high when a young black girl is killed and a race begins between the cops and the BLA to find the supposed killer. Sam teams up with Fletcher Bellow, a young detective in 1971, who is his captain in 2008.

With a life-and-death hostage situation paralleling own strange circumstances, Sam Tyler is ordered with the rest of the 1-2-5 to diffuse a potentially deadly face-off at a hospital where a crazed gunman is trying to force a doctor to reverse a dangerous operation on the renegade's brother, and the squad are all targeted in the gunman's cross-hairs.

When the baby of a prominent family is kidnapped for ransom, Sam Tyler is shocked to discover that his own father, Vic Tyler, may be involved. Sam's discovery leads him to face some deep dark secrets about his childhood.

Lt. Hunt's squad and Lt. Nunzio's crew renew their long-standing rivalry due to a bank robbery with a connection to the Russian mob. Meanwhile, Sam is attracted to a pretty, young social worker.

Ray's shady brother may have played a role in the number of recent decapitations in which the squad is investigating. but other suspects pop up too, such as a biker, a Wall Street executive and badly disfigured man.

When rock star Sebastian Grace receives a death threat, Sam and Chris are exposed to the rock-and-roll lifestyle that includes one of Grace's groupies called "Rocket Girl." Things really get strange when Chris finds Sebastian and Rocket Girl waiting for UFOs in the swampy area of New Jersey known as the Meadowlands.

NYC Councilman Bobby Prince shares a secret with Sam after Prince is caught with a hooker during a raid on a motel. The secret leaves Sam with even more questions as well as the realization he has more in common with Prince than he had ever thought. Meanwhile, Gene Hunt locks down the precinct after a shooting, Maria and her father confront each other, and Ray's wife, Denise, adds tension to the drama.

The detectives at the 1-2-5 investigate the murder of newspaper journalist Joey Conway. Sam is convinced the killer is the same criminal from a case in 2008. He is motivated by flashbacks to the case, which puts him at odds with his coworkers who think someone else is guilty.

When the Weather Underground sends the department bomb threats on the anniversary of their leader's death, Lt. Hunt takes the threats personally. As Sam, Annie and Lt. Hunt investigate, they soon find a connection between a student and Pat Olsen, a professor.

When a Pan Atlantic air hostess is found dead, Annie volunteers to go undercover in order to solve a series of homicides. Annie, Sam and Ray, working undercover, are able to find an unlikely suspect.

When Sam goes undercover as a gang member, he has to reveal a dark side in order to keep his cover. While undercover, Sam is pushed to the limit when he is ordered to bind and shoot a woman.

Crime hits home after Ray and Chris are shot by a gang member. When the shooter is found dead near a bar, Sam becomes the lead suspect since he has been working undercover as part of the gang. With evidence mounting against him, Sam must search for the real killer before it's too late for him

When Sam Tyler receives a phone call that presents him with three tasks he must perform so he can return to his life in the future, he must weigh his alternatives and decide whether to stay in 1973 or go home.