Whilst Brian faces disciplinary action for a death in custody from his past the rest of the team discover the gun that killed drug-trafficker Christian Highsmith in 1998. The same gun killed a young boy, Danny Bossano, in 1982 in Gibraltar, a port of call for the ships on which Christian allegedly hid the drugs. The UCOS team go to Gibraltar to meet Superintendent Cruz, the local cold case chief, who believes Danny was part of the dope-smuggling gang. The fact that the gun was army issue also leads to the evasive commander Adam Sinclair whilst Sandra quizzes casino owner Harry Truman, who had been a friend of Christian. Gerry and Steve visit Truman's casino but accidentally get locked in a shipping container whilst shipping agent Gordon Fletcher, an associate of Truman, is found dead.

The discrediting of DI Hennessy as corrupt throws into a new light some of his more high profile arrests, including that of crooked businessman Edward Monroe, jailed for killing oncologist Dr Caldwell, whom he blames for the death of his daughter. For Sasha, now alone after discovering her husband's infidelity, the case is painful as she has always believed Monroe murdered her former colleague DC Tyler. As fresh interviews are conducted, it turns out that Caldwell was not a pleasant man and had made enemies of his colleague, Dr Hallerman, passed over for honours; Daniella Yates, director of a cancer charity who was having an affair with Caldwell; and Caldwell's duped wife. However a statement from Grace, a hotel employee of Monroe, throws a more humane light upon the convicted killer, as well as giving Sasha some closure for Tyler's death. Steve, meanwhile finds himself billeting his son Stewie, who has run away from home.