When NYPD detective Sam Tyler is hit by a car while searching for his missing partner, Maya Daniels, he finds himself in 1973 once regaining consciousness. There, he works for precinct boss Lieutenant Gene Hunt, a man not afraid to bend the law. Now Sam must work to find why he is mysteriously in the past.
After hearing a radio announcement saying Elvis will be performing in concert, Sam remains unsure as to how he found himself 30 years in the past. However, he must put his own predicament aside after he is put on a case involving fatal robberies at a check cashing store.
After a Vietnam veteran is killed upon his return from overseas, Lieutenant Hunt demands a quick resolve to this complicated case. As Sam and the detectives look into the vet's homicide, Tyler uncovers clues about his family's past.
Sam gets pulled back into his childhood after he must deal with gangster Nick Profaci's assault on a young woman, who turns out to be his mother in 1973. As Sam tries to save her, he learns about two unsolved murders linked to Profaci and his boss, Elliot Casso.
A racial fight is triggered between the African American and Puerto Ricans when a young African American girl is found dead. Sam is assigned to the case and thinks about his mentor in 2008 in order to get an edge on the case.
Sam Tyler finds it difficult to separate a life-and-death hostage situation taking place at County Hospital from his own strange circumstances. Stressed about a phone call where he seems to hear his mother's voice -- in 2008 -- begging him to wake up, he and the rest of the squad are ordered to diffuse a potentially explosive face-off in which a crazed gunman is trying to force a doctor to reverse a dangerous operation on the renegade's brother. Sam, Ray, Annie and Lt. Hunt are all targeted in the madman's cross hairs, but as Sam and the 1-2-5 attempt to resolve the confrontation, he suddenly realizes that the clock ticking on the gunman's demands parallels another deadline that could spell the end of Sam Tyler.
Sam Tyler is shocked to discover that his own father, Vic Tyler, may have been involved in the kidnapping of a baby from a prominent family. Sam hasn't seen his father since 1973 when he was only four, having disappeared under mysterious circumstances. Was his reason of being sent back in time was to stop his father from leaving his family? Vic makes an agreement to work with the squad so they can retrieve the missing baby, but the deeper Sam gets into his father's world, the closer he comes to uncovering some dark, sordid truths, compelling him to face long suppressed childhood memories. His discovery culminates in a face-to-face which will leave Sam changed forever.
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.
Sam and Chris are pulled inside the world of a rock star who is receiving death threats, and a spacey groupie and an FBI agent known as "The Sorcerer" may be involved. A twist in the investigation has Chris wondering if he has found a way to return to his time.
Sam discovers a shocking secret after a councilman is caught with a hooker when the police conduct a raid on a motel room. Meanwhile, Lt. Hunt puts the precinct on lockdown following a chaotic shooting inside the building. Hunt has a confrontation with his daughter, Maria.
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, matching up with two previously murdered girls, Annie, who happens to closely resemble the dead girl, volunteers to go undercover in order to help solve the case. Assisted by the girl's extensive diary, Annie, along with Sam and Ray, work undercover to find the perp.
Sam goes undercover to infiltrate a violent Irish gang, and has to reveal a dark side in order to keep his cover. The situation gets twisted, though, when it turns out that the sister of the gang leader is actually a neighbor who occasionally babysat him in his youth. The connection to his youth once more challenges Sam's perception of his situation.
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.
Sam receives an offer to return to 2008 from a mystery caller, but he finds himself confused over a certain set of circumstances which makes him wonder if he even wants to do this.