Jack and Strickland form an unlikely alliance that sees them break the rules to find out why a British soldier died while taking part in a Ministry of Defence medical trial. With three of the man's colleagues now suffering from psychological, drink or memory problems, the team consider whether he died as a result of the experiment, or if he was a victim of an Army cover-up.

As the series comes to an end, the desiccated, neatly carved-up body of a butcher who disappeared more than 30 years ago is discovered, leading Sandra Pullman and her boys to pay a visit to Smithfield meat market in London, the original murder scene. It's a pity the butcher is dead, because he has a few questions to answer about why the body of a doctor was found hanging from a meat hook all those years ago at the back of his market stall. It's an episode peppered with revelations. We discover Gerry Standing's very interesting family history - he's descended from French Huguenots and is thus tormented by his colleagues for being a fake cockney, much to his annoyance: "Je suis ang-bleedingglais!" he protests. But it's Sandra who has to face up to a startling personal disclosure that has a direct bearing on the case and also throws a whole new light on her father. For once Gerry, Brian and Jack are left completely speechless.

After being released from jail, notorious paedophile John Davies admits to the abduction and murder of a child 25 years previously. Suspicious of his motives, the detectives reopen the unsolved case in an unusual attempt to prove his innocence, reinvestigating the boy's disappearance while on a demonstration with his left-wing activist parents. Guest starring Adrian Schiller (Being Human) and Samantha Bond (Outnumbered)

The team reopen the case of zookeeper Zac Halsey, originally thought to have been mauled to death by a tiger, when blood evidence discovered in his lodgings suggests he was killed before he was found in the tiger's enclosure.