1948: Post-war Britain and 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. The killer strikes again and the team discover another linked murder from the 1960s. A further victim is found nailed to the floor - this time in a nursing home for the elderly. Is the Second World War the linking factor? The team must find the next possible victim but no-one knows where he is.
Two 5-year-old twins are taken from their home 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. Meanwhile, Jason has disappeared and a murder has taken place.
The Team effectively proves that dead man was a murder victim, not a suicide and narrows their prime suspects to three inmates at the halfway house where the crime was committed. Two of them have ties to a shady jazz club owner named Phil Brown. After the murder weapon, which is linked to other execution type murders, is stolen from Frankie's lab, Brown is murdered with it.
The body of DS Dave Marvin is found with his throat slashed in a parking lot that has connections to the case. Although there were no signs of sexual activity as in the other cases, someone tried to carve letters into his body but was evidently interrupted. A very embittered Raymond Carstairs reveals that his father, one of the victims, not only was headmaster at a children's school but abused children too, even his own son. Grace suspects that the semen found on the victims may have been purchased on-line. As the case builds to a resolution Bulmer and Boyd clash.
A fourth schizophrenic young woman falls under the spell of the shadowy religious figure known as "The Shepherd," who exploits her paranoia and manipulates her toward murdering psychiatrist Matt Carney, brother of her doctor, David Carney.