Thomas Kell is an agent with MI-6.
Well, he was. For nearly two decades he has been an agent in very good standing with that branch of British Intelligence. Then the apprehension of a British citizen turned likely ISIL soldier goes a tad haywire. Kell led the group tracking him down and pulling him from the Middle East. Since British law prohibits torture and the man would not talk, somehow without Kell really liking it but not doing much to stop it, the man is handed over to the CIA who then used other countries to assist in convincing the man to talk.
Kell might not have liked it but he did not like the idea of a traitor going free so he did not lose much sleep over it. He did lose his job, though, when the British press got wind of the event and looked deeply into it and the powers that be decided that a scapegoat was vital so they could keep their positions. Suddenly Kell was out the door and at a total loss for what to do.
From the time he had entered the adult world, Thomas Kell has been a MI-6 agent. He had no idea what else he might do. He had never thought about it because being a MI-6 agent was all he had been and all he wanted to be. And he was damn good at it. And how in the heck do you take the skills that made you so good as an operative and translate them into something that would earn you a living in the civilian world. More importantly to Kell, how do you find anything remotely as interesting and challenging.
Adding to Kell's troubles is his marriage. He truly loved his wife and she was sticking around so apparently there was still affection from her but the relationship was on very rocky ground made far worse by the fact that he was so terribly unhappy out in the cold with work.
Six months after his boot, a situation would arise and someone not officially in the organization would be needed to handle a very sticky situation. Kell would be perfect for it. Kell would himself love the chance to work his way back inside. It was not a certainty. Still, it was a chance.