A detective chief inspector from 2006 is investigating a serial killer when he is knocked over by a speeding car. Waking up, he finds himself mysteriously transported back in time to 1973. Initially struggling to come to terms with his situation, he has to come to terms with the old-fashioned technology and attitude of the day, while figuring out how he came to be trapped in the past. The actor John Simm described the show as "a cross between Back to the Future and The Sweeney", and it makes effective use of the disorientation of the unwitting time traveller while taking a post-modern romp through 1970s fashions and technologies, with due tribute paid to the classic police dramas of the day to fashion a truly unique programme.
DCI Sam Tyler, in pursuit of a serial killer, is knocked unconscious by a car and wakes up in 1973, where he is a newly-transferred detective inspector working at the same station. Initially refusing to accept his situation, he finds it difficult to adjust to the attitudes and technology of the day. Investigating a murder in 1973, he recognizes a possible connection to events in the present day.
When Sam discovers that DCI Hunt plans to plant evidence to secure the conviction of an armed robber, he decides to release him.
Sam is called to a murder at a textile mill, the site of his bachelor flat in 2006. DCI Hunt suspects an outspoken union rep, but Sam tries to use his knowledge of forensic science to find the killer.
Sam arrests the henchman of Stephen Warren, a local Mr Big, when he learns the whole of CID are on his payroll. Sam also visits his childhood home to interact with his mother (using name of "Bolan", a reference to the glam rock star Marc Bolan), who was being threatened by the landlord, one of Warren's men. Sam tries to help her by giving her some money and she mis-understands his gesture. When Sam tries to go all out after Warren, he finds himself in fix when Warren sets him up in a honey trap. The girl involved in the honey trap sees the error of her ways only to be murdered by Warren. Gene disagrees with Ray who says that if they don't play the game people get hurt and helps Sam bring down Warren finally putting his guilty conscience to rest. Sam also attempts to persuade Gene to swap his Red Rum for Tyler's Proud Percy in the Grand National sweepstakes.
A football fan is killed, and Sam fears further violence in the run up to an important local derby. He and DCI Hunt mount an undercover operation in a pub to catch the suspects. Sam realises that the match is the last one he attended with his father.
Sam, convinced that his life-support system is about to be switched off in his own time, convinces himself that resolving a hostage situation will allow him to wake up.
A young man being held for a minor drugs offence dies in custody at the station while Gene and Sam are out. While Gene tries to protect his team, Sam struggles to conduct a proper investigation into the death. After alienating all his colleagues, he is able to determine that one of them forced him to take cocaine, causing a heart attack, and that the others had been covering.
Sam encounters his parents again in 1973; he believes that if he keeps his father from running away he will be awoken from his apparent coma. The closer he gets to apprehending his father, the more he hears sounds that tell him he's close to waking up. However, rather than traumatizing his family more by arresting his father, he instead lets him run away. Flashbacks throughout the entire series are revealed to have been from young Sam's memories of this incident.