A US diplomat is shot dead in a central London square, forcing the team to work closely with the embassy and the FBI and to overcome jurisdictional hurdles.

While two care home staff are chasing Kevin McDowd, a young man with learning disabilities and sociopathic tendencies, Nikki and Jack examine a fatal road accident where the deceased is Kevin McDowd's mother. The investigation led by police sergeant Button should be straightforward, but the forensic evidence suggests foul play.