In 1948, post-war Britain, an ex-soldier has been found dead with his skull nailed to the floor. Now this case has been re-opened due to media attention and pressure created by his grandson.

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.

After shooting an unarmed man in the raid on the nursery, Mel is suspended from the case pending an inquiry. Grace and Boyd, meanwhile, have more questions for Roper—but the doctor's in no position to answer them.

After Sam is released from prison, a man is murdered in the hostel where he stays. Soon he is a suspect. But it looks as though Sam was in fact the intended victim. On the surface Sam appears to be a devoted family man, but he has a hidden past which the team can only speculate about.

When the gun stolen from Frankie is used in another murder, the Squad suspect the victim's gopher. It is revealed the first murder could have been a case of mistaken identity. Frankie and Grace begin to get creeped out by the effects of Boyd's Anger Management!

A sadomasochistic serial killer is at large. The killings are brutal and the victims are found with the word 'Sorry' carved in their backs.

When the killer strikes at the heart of the team, the investigation takes a new track and the motive for the murders takes on a very different meaning.

A young woman who has psychiatric problems has killed her family through an arson attack on her family home. She claims she was incited to do this by a mysterious figure known only as 'The Shepherd'.

Boyd and the team continue to investigate the arson case and the other two other possible crime connections to 'The Shepherd'. Will the team discover this person's true identity or will it all turn out to be a part of the imagination of a mentally ill woman?