High-flying US cop Bill Hixon arrives in Boston to shake up the East Lincolnshire Constabulary, but when the coldest of cold cases turns up, it is Bill who finds himself shaken up.

Bill tries to reboot his authority by getting his team to focus on the difficult unsolved crimes, including that of the ‘Boston Bandit’ who was responsible for a series of post office robberies and a murder. Meanwhile, Bill’s professional relationship with Mary gets personal, Oleg pressures Muriel into betraying Bill, and Kelsey succeeds in getting Bill to finally engage with his dead wife’s memory.

Bill launches a PR offensive when a leaked list of potential redundancies turns his team against him, and courts Boston’s old folks by claiming his new approach will make them safer.

The discovery of a pyre of bodies puts Bill on the hunt for Frank McGill, a hitman notorious for his weapon of choice: a nail-gun. However, having been put into witness protection years earlier after turning in other members of the Maguire gang which he worked for, Frank is now off-grid. Bill sets his sights on tracking him down. Meanwhile, DS Blair is released on bail after information is leaked regarding the unlawful obtaining of key DNA, Commissioner Metcalfe discusses the financial benefits of selling off a disused training facility and the pressure mounts for Bill to resign.