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.

When the body of a young local man is found between two rival farms, a bitter feud is reignited. With the two families having been enemies for over 30 years, the team must deduce which of them is responsible. Bill and Muriel find themselves on opposite sides and both are determined to prove themselves correct. Bill wrestles with introducing Mary to Kelsey. Kelsey makes a new friend at the local school. Lydia resents doing Bill's dirty work as the redundancies commence.

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.