MI-5, aussi surnommé "Five", n'est autre que le service secret britannique fondé pour protéger la sécurité nationale des menaces intérieures. Leurs missions : combattre le crime organisé, démanteler les trafics d'armes et autres réseaux terroristes...

La saison 3 est marquée par des dissensions au sein de l'équipe du MI-5, et la disparition de Tom Quinn, suite à un coup monté.

It is all change in the clandestine world of MI5 when new agent, Adam Carter, joins the team. Tom has been branded a traitor and has disappeared, while Harry is in hospital, seriously wounded, and there is dissension in the ranks. Danny and Zoe seek to clear Tom's name despite the fact he appears guilty and may even be dead. Will Tom, missing since shooting Harry, re-emerge to prove himself an innocent victim of a set-up? And will former MI6 agent Adam Carter, the new edition to the team, provide the drive and enthusiasm so apparently missing? If his track record and the reputation that precedes him are anything to go by, there are going to be plenty of fireworks.