It's 1969, and things have taken a darker turn for the old Cowley team. With the gang now scattered across Oxfordshire, it takes a series of brutal crimes to reunite them. Now joined by former adversary DCI Ronnie Box and his sidekick DS Alan Jago, the gang must battle crime and corruption to finally solve their greatest challenge, which is to find the truth behind George Fancy's death.

As 1968 dawns, a year of change promises to disrupt the private and professional lives of Endeavour Morse and his colleagues. Endeavour's recent promotion leads him to reluctantly mentor new recruit Fancy, while Thursday is beginning to think about life after the police. But in Oxford, crime never sleeps, and the team continues to be challenged by mysterious and intriguing cases of murder, greed, and deception. Meanwhile, the rise of organized crime...

Spring 1972 brings with it a new start for Morse, having taken time out to deal with the drink. His return to CID coincides with a gruesome discovery in a college garden. The investigation leads him and Thursday to a celebrated orchestra and when a second tragedy hits, they uncover a web of secrets at the company's core.

Endeavour and his colleagues are entering a new decade and era of change. Opening on New Year's Eve 1969, normal order has been resumed and the team reunited at Castle Gate CID, with Chief Superintendent Bright back in charge. However, the events of the past year have left their mark. Old friendships will be challenged, and new relationships will blossom.

1971 has barely just begun and CID is still reeling from the events of the past year. Any hope of remaining on light duties has vanished and the team find themselves right back in the thick of it.

Following the dramatic finale of season three, Oxford's finest are picking up the pieces of their personal and professional lives. DC Endeavour Morse awaits the result of his Sergeant's Exam, while DI Fred Thursday and his wife Win struggle to cope with their children flying the nest. Sam gone to the Army and the traumatized Joan, gone off to who knows where? There are even greater challenges still to face.