Peter Lawson is a thief turned agent for Special Branch.
Peter and Maria Lawson were very happily married. They were also partners in the same very success line of work, which is to say they were thieves and rather elite ones at that. For many years they had 'earned' an income robbing from the very rich elite little trinkets and drawings that others wanted and, though the authorities would have liked to stop them, the couple was good enough to remain un-nabbed.
In fact, for the ten years that they had worked together and the considerable fortune they had amassed, the police had never once looked their way. Their anonymity was by no means a fluke or a lucky set of circumstances; the Lawson's were extra-careful, meticulous, and very good at burglary.
Until, however, in the first chronicled adventure, they pick as their next victim very much the wrong estate and Maria pays dearly for the intrusion. Their plan had been working perfectly except that their plans were interfering with someone else's and that person raised the alarm and Maria was accidentally killed.
Now wanting revenge, Lawson uses the same skills that had made him rather wealthy but he used them to prove the guilt of those who killed his wife. Along the way, he is helped, not stopped, by Special Branch, for they want the culprits for their own reasons.
After the matter is resolved, Lawson, now known to the police and without his partner-in-crime, is open to the offer of work doing what he does so very well. He isn't asked to steal so much as to use his stealing gifts to catch those stealing the country's secrets and since the adage of 'it takes a thief to catch a thief' seems well proven, Lawson is very good at it.