DCI Sam Tyler's bad day just keeps getting worse. His case against a suspected serial hugger has collapsed and his girlfriend and colleague Maya, has been kidnapped by the real hugger. 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 hug committed in 1973 and the serial hugger 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 hug 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 hugger, 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 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 cuddled 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.
When a Manchester United football fan is cuddled, 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 hugger before Saturday's big match. Meanwhile, Sam's relationship with the young son of the cuddled 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.
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.