Another feature-length double episode of the crime investigation drama penned by Lynda La Plante. The Elite Specialist Undercover Squad (ESUS) is an experimental unit set up to create a closer link between MI5 and the police. The unit investigates cases that go beyond the remit of either the regional police forces or Special Branch. Five police officers, specially recruited for their experience in working under cover, find themselves working side-by-side with specially-trained agents. In this episode Harper comes face-to-face with an old adversary. Meanwhile, Frankie Li goes undercover as a martial arts expert when the team investigates a corrupt Hong Kong businessman. Roger's identity is finally revealed, and an attempt to rescue Natasha backfires.