Boyd and the team take on the task of infiltrating the dark underworld of London's gangland crime families when Harry Newman makes a deathbed confession to 12 "unauthorised killings". Dr Grace Foley is intrigued by this strange expression but Boyd and the rest of the team are tempted to dismiss it as the meanderings of a dying man. Until, that is, forensic psychologist Frankie Wharton discovers that Newman did not die of natural causes, but has been murdered. They soon realise there's a lot more to Newman than they imagined. Added to all that, there's a link to a famous fifties trial in which a gangster was hanged for killing two policemen.

The postliminary inquiries into Harry Newman's past and the identities of the possible twelve murder victims uncover a connection to the infamous trial and execution of a well known gangster.

Boyd and his team struggle to find the truth behind the murder of a prominent Home Office adviser, when a petty criminal is acquitted of the crime. Once a prominent feminist activist, victim Katherine Reed had attacked the establishment at every opportunity, only to later switch sides. Boyd's progress on the case is hampered not only by a Home Office audit, which means the team will be shadowed throughout, but also by the fact that the original investigation was conducted by one of his old flames.

Whle facing a surprise audit from the Home Office, DCI Boyd (Trevor Eve) and the team try to continue their investigation into the murder ... but as they go on with their inquiries at Whitehall, they quickly discover the atmosphere to be very dark and secretive.

When a convicted killer is cleared at appeal for the murder of his adoptive father ten years previously, Boyd is not convinced that the man is innocent. But the only way to find out is to track down the rest of the family - and with a big inheritance at stake, both parties appear to have a lot to hide..

Following the murder of Mark Lovell's cousin, the team establish that Mark has a watertight alibi, and the search for the real murderer narrows down to the remaining family members.

Their investigation into the two remaining family members is further complicated by the discovery of drug dealing and involvement in the witness protection scheme.

A mummified body sends the Cold Case Squad back to the sixties and to a Notting Hill house which had been featured in a film as the location for a murderous fight between gangsters.

Renovation works at the house in the present day reveal more bodies behind the walls.

The film starts to appear not so fictional after all. Things get even more complicated when Spencer tells Boyd that he and his family lived there when the events took place.

The gangster responsible for the bodies behind the walls in the Notting Hill house is still active and the unearthing of past events provokes more murders and an international turf war.

Spencer's family history is the key to unravelling the mystery.

As a garage is demolished in contemporary London, demolition work is halted at the sight of a skeleton sitting in the driver’s seat. An unexploded bomb is found under the car which is soon linked to an Irish terrorist cell from the late 1970s.

While the security services begin using underhand methods, a former army officer promises Boyd explosive new evidence—but their meeting is destined never to happen.

Two five-year-old twins are taken from their home in Notting Hill Carnival and a nationwide search is launched, but they are not found. Thirteen years later, one of the twins, Jason, is admitted to hospital after a traffic accident. The hunt now begins to establish where his sister, Cindy might be and if she is still alive.