The team reinvestigate the disappearance of two young political activists.

A recent rape is linked to two similar attacks at a chocolate factory 10 years previously, leaving the team no choice but to reopen all three cases. The detectives uncover news of an unhappy workforce, learning of an alleged suicide, and their investigation becomes even more complicated when they discover a severed finger was found in one of the company's snacks. After a visit to the doctor, Gerry decides to quit smoking, and instead of resorting to nicotine patches or gum, he enlists Sandra to help him quit

When graffiti appears around London by an unknown person claiming to have murdered a celebrated street artist, the team reopens the four-year-old case. Originally part of a gang, the victim went solo when he received support from a wealthy art patron, leaving the detectives suspicious of his resentful former crew members. Meanwhile, Brian signs up to Twitter to chart the gritty life of a lawman, much to his colleagues' displeasure.

The team become concerned that undercover police operations may have been compromised when the body of a computer expert who worked with the Met turns up under a false name at a teaching hospital.