DCI Sam Tyler's bad day just keeps getting worse. His case against a suspected serial killer has collapsed and his girlfriend and colleague Maya, has been kidnapped by the real killer. Worse is to come, he is knocked down by a car, when he wakes up - he appears to be in the year 1973. Confused, he's convinced he either dreaming or hallucinating and he's finding it difficult to believe all around him is real, including his new fellow officers. Sam discovers there is a link between a murder committed in 1973 and the serial killer who has kidnapped Maya - in 2006.
Sam’s policing method from 2006 is causing conflict with Gene Hunt, his DCI who’s policing method is rooted firmly in 1973. Unable to determine whether he really is in 1973 or a coma, he gets on with his job. When Kim Trent - high profile criminal is arrested, Sam discovers that Gene has planted evidence on him to secure a conviction, Sam intervenes and releases Trent to Hunt’s fury. Sam argues that even Trent has human rights, Gene tells him he’s never fitted up anyone who didn’t deserve it. Between them, they set out to nail Trent using Gene’s unorthodox methods and Sam’s 21st century policing methods.
Sam is called to a murder at a textiles factory - which he is shocked to discover will be his own home in the 21st century. He puts his knowledge of forensics into use to find the killer, but Gene has a simpler approach - the first suspect to speak did it. As battle lines are drawn across CID, it becomes increasingly clear that the Seventies method of detection may in this case be right.
Sam discovers many of his colleagues are taking bribes from a local gangster, and resolves to put a stop to police corruption. However, a skeleton in Gene's cupboard makes him reluctant to join the crusade, and his fears are underlined when a young woman mixed up in the case meets a grisly end. The 21st-century detective also can't resist tracking down his mother, who is facing money worries.
When a Manchester United football fan is murdered, police suspect a rival City fan's to blame. The incident raises fears of a riot between supporters of the two teams, whose derby match is taking place at the weekend. Sam, Gene and WPC Annie Cartwright go undercover to gather evidence. But Sam worries they'll fail to find the killer before Saturday's big match. Meanwhile, Sam's relationship with the young son of the murdered man brings memories of his own father flooding back.
Sam hears his mother's voice saying goodbye, and worries that doctors in 2006 are about to turn his life support system off. When a hostage situation crops up, he becomes convinced that resolving the crisis without a single casualty will cause him to wake up in his own time. But Gene is determined to go in with all guns blazing, putting everyone's safety at risk.
A minor drug dealer and flasher dies while in police custody and Sam suspects a cover-up within the unit. He demands a full investigation, though it seems none of the other officers is prepared to help. To Gene's relief, the pathologist's report appears to put everyone in the clear, but when the truth about the criminal's demise emerges, it's up to the modern-day cop to decide what action is appropriate.
The investigation into the murder of a bookies' clerk leads the team to 29-year-old Vic Tyler - who Sam is shocked to discover is his own father. He realises the time when he and his mother were abandoned is coming up, and desperately tries to persuade the small-time gambler not to leave his wife and child. Meanwhile, Gene sees an opportunity to bring down a crime syndicate.
Sam, still stuck in 1977, comes face to face with a man he knows convicted murder in 2006, and helped set up the case for. Sam's chosen methods of police work on the case confound his colleagues.
At a loss to discover the guilty party in a spate of armed robberies, A-Division goes to prison to escort notorious safe-cracker Dickie Fingers back to CID. However, as they transport him out of the gaol, they are set upon by a dangerous gang who will stop at nothing to get their hands on Fingers.
When a bomb warning is reported, A-Division go on red alert. According to the caller, the IRA has brought their mainland bombing campaign to Manchester. Sam, however, based on his knowledge from the future, doesn't believe it's the IRA, which puts him at odds with Gene and the rest of the squad, especially when his initial presumption that it's a hoax gets Ray injured.
When the body of a beautiful young woman is found in an abandoned lot, Gene fears a serial killer he believed sent down in the late Sixties may still be at large. For Sam, the victim, a Beauvoir beauty rep like his Aunt Heather, triggers vivid recollections of childhood. The case leads to the seedy side of private wife-swapping parties.
When a desperate man whose family has been kidnapped attempts to hang himself in the division offices, the crew is forced to re-examine an old case believed closed. The result is a Rashomon-like overview of the case interviews and information, even as Sam attempts to deal with a accidental drug overdose to his comatose body in the here-and-now future.
Sam comes face to face with the bigotry prevalent in the 70s when a Pakistani immigrant is shot, with a small amount of heroin found on his body. The automatic assumptions of Hunt, as well as Ray and the others rubs Sam the wrong way. Gene is more concerned witht he recently increased heroin supply which appears to tie to the wounded Paki, and is resulting in a slew of dead junkies being found around the area.
When a case against a lowlife goes south, Gene gets totally blitzed. When he wakes up the next morning, he's in the presence of the lowlife's corpse, covered with the victim's blood, and his gun has apparently been used in the murder. As the team tries to prove Gene's innocence, a temporary DCI is put in charge of the unit. The result leads to a shocking discovery about Sam's condition.
Sam is confronted with a fateful decision: to abandon his mates and return to the present, or to stand with them and take life as it is in 1973.