Joshua 'Josh' Thane is a freelance assassin.
He kills people for money and he is very good at it. If the client has a special condition in mind, such as making it look like an accident or a mugging, he is more than qualified to give that kind of service though he charges a good deal for it. If his clients were inclined to talk about it, which naturally they are not, they would give high praise for his results.
Exactly how many years he has been at this not-so-nice occupation is not specified but it is certainly has been over a decade which makes Thane well into his 30's and not planning on changing anything in his life anytime soon.
At least he wasn't until the assignment we first watch him on - kill a middle aged woman in her home and get from her house a hard drive. Simple. Except when he arrived in the late evening to take care of the matter, she was already murdered and the hard drive gone. And then there was the matter of her young son which Thane was not told about and should have been and Thane should not have left a witness but chose to do so.
And that is when Thane's life takes a drastic change and when he will find himself working with, although not particularly willingly, with Inger McCallen, the beautiful woman with "the pale colouring, the copper-coloured hair, and flinty manner" who joined in the investigation but with a difference. McCallen was not police. She was MI-5 which meant that the murder victim was somebody important to national security or the hard drive was or both. It also meant that Thane's career was about to take a major turn.
Thane is smart enough to know that MI-5's job was not to catch and incarcerate criminals, even killers like Thane. It was to protect the security of the country and that often meant dealing with people like Thane and once they have hold of a craftsman tool as honed and sharp as someone like Thane, they aren't likely to let it go.