In the wake of a devastating double homicide by one of their own, the army takes matters into their own hands to hunt down the man responsible. In a tense race to stop a civilian terrorist attack, it's down to an elite team to take on the shadowy forces that lie in wait.

When a British Special Forces soldier goes rogue, carrying out an attack at a US Army base in Syria leading to the death of two men, ex-MI5 controller Charlotte Button is hired to work out what his plan is and to take him down before he can carry it out.

When the man returns to the UK, it looks like the threat will be a lot closer to home. Charlotte puts her best man on the case - hitman Lex Harper. It's up to Lex to assemble a crack team and get to the soldier before he carries out what could be a massive terrorist attack. But Charlotte might require Lex's help with a more personal problem, too. As ex-MI5, Charlotte knows a lot of information about a lot of important people. And now someone has gone to extreme lengths to get to that information by breaking into her safe deposit box in a Hatton Garden-style armed robbery.

While it's down to Lex to prevent a potential terrorist attack on British soil, Charlotte must work out who is after her and what they want.