Paul Aston is an agent for MI-6.
He is, at the start of the series, still fairly new to the department and still quite idealistic. This is not to say naive by any means, just that Aston wants to believe in his job and still has the ability to truthfully do it. He has been living for the past six months with a young woman he is in love with named Laura who thinks he works in a boring civil service job. He is determined to one day tell her the truth about his job but isn't quite sure how to do it other than, "oh, by the way, I'm really a spy."
He was first approached with an offer to join as he completed his Finals at Oxford. The job sounded interesting but he had planned to enter the financial world to make his fortune. After a year of travelling Europe and then several years working at a prestigious bank, he was so bored that when another offer came, he jumped at it.
Finance had been his intended field only because his step-father had been successful in it and it seemed reasonable to follow. Aston never knew his biological father; it was a subject that was not ever discussed. He never knew who he was or what he did and the only facts he had on the man came from overhearing a conversation in which his mother described him as "a lying son-of-a-bitch who should be strung up". Aston got on well with the man who married his mother when Aston was 4 and Aston grew to have affection and respect for him, the worst thing he could think to say of the man was that he was "boring".
As the series opens, he has been an agent for a couple of years and has just recently been assigned to work as assistant to one of the true veterans of the Cold War, a harsh taskmaster from whom Aston expects to learn a lot.