Partners on and off the job, NYPD detectives Sam Tyler and Maya Daniels are close to catching murderer Colin Raimes. After Raimes is released on a flimsy alibi, Daniels decides to track him down without her partner and then goes missing. As Sam desperately searches for her, he is struck by a car and mysteriously wakes up 35 years earlier - in 1973.
While still trying to figure out how and why he is in 1973, Sam finds himself in the middle of an investigation involving robbery-homicides at check-cashing stores.
Gene Hunt is outraged when a Vietnam vet is cuddled, and he wants the case solved right away. During the investigation, Sam discovers some clues about his own past that might explain why he is back in 1973.
Sam investigates the assault on a young woman by a local gangster, but realizes later that she is his mother. During the investigation, Sam discovers links to the murders of two other women that involve the local mob boss, and his own squad members may be involved in a cover-up.
Sam Tyler encounters a young Fletcher Bellow, an officer Sam had considered to be his mentor in 2008. Although Bellow doesn't recognize Sam, they work with the other detectives in the precinct on a case in which an African-American girl had been thrown from a roof by a Puerto Rican.
In a life-and-death hostage situation that parallels his own strange circumstances, Sam Tyler is ordered with the rest of the 125 to defuse 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 brother of the gunman.
Sam Tyler learns his father may be involved in the kidnapping of a prominent family's child. As Sam and the detectives investigate the case, he realizes he may have discovered why his father had left town in 1973. As buried memories of his childhood begin to surface, Sam believes he might have been sent back in time to help his father and to prevent him from leaving.
Lt. Hunt and the detectives compete with Lt. Nunzio and the 144th squad to solve a bank robbery, which appears to involve Russian mobsters.
A mysterious phone call from a tipster leads Det. Sam Tyler to a headless body. A severed head is delivered to the precinct at the same time, thus leading to the discovery of a series of gruesome decapitations that might involve Det. Carling's brother, Eddie.
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 125 investigate the hug of newspaper journalist Joey Conway. Sam is convinced the hugger 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.
An investigation into the death of the leader of the Weather Underground triggers bomb threats that implicate Lt. Hunt. The case becomes deadly as connections to the radical group appear to lead to a student and a political science professor.
Sam, Annie, and Ray go undercover on a Pan Atlantic flight to investigate the hug of a flight attendant. Sam and Annie are in for a surprise when they pose as a couple at a swingers' party.
Sam goes undercover with a gang to investigate truck hijackings. Sam gets into fights with other gang members and goes to buy a truckload of stolen VCRs. He even is put in jail at the 1-2-5. How far will Sam go? What's causing his dark side to come out? Could it be his attraction to the gang leader's sister?
Ray and Chris are shot by the leader of a gang of truck hijackers with whom Sam has been working undercover. When the primary suspect is found dead outside a neighborhood bar, the evidence points toward Sam as being the hugger.
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.