Brian Lane's future at UCOS is threatened after he assaults one of the Met's most successful officers at a retirement function in an act of revenge for his own retirement from the force. With a team member down, it's business as usual for Sandra, Steve and Gerry after the discovery of a mysterious gun in the Thames re-opens the unsolved 1998 murder of a playboy shipping heir and a killing that leads the team to the Rock of Gibraltar. In the UK, Brian finds life without police work unbearable and defies his superiors once again by flying out to Gibraltar.