Simon Quarry is a freelance agent.
He thinks of himself as a publisher of books, which really was all he wanted to be and it is his profession when the series starts. He is good at it; successful and well respected. His firm is by no means the biggest but it is of moderate size and even bigger reputation. He certainly has no desire for it to change for he has a good life with a much loved wife and two daughters and a great deal to be thankful for.
Then terrorists strike an airliner in Athens and Quarry's wife and daughters are caught in the slaughter. Even more horribly, they are caught on camera dying. When he tries everything a successful, civilized man can do to get justice and is just met with fear, indecision, greed, and apathy, he decides he had to try something more drastic.
Dropping the habits of a lifetime to become as blood-thirsty as those he needs to hunt is not easy but it was what Quarry has to do. And he proves he can be very good at it.
His initial job is made a bit easier since the terrorists, feeling themselves safe from repercussions, are not exactly hiding. After training and getting himself in shape physically and mentally, he goes after them.
That is the crux of the first book. The remaining two books deal with the aftermath of his actions, the reputation and recriminations and even reward that he faces from what he has accomplished, and the continuing need for someone with his newly developed skills and his attitude that he has nothing much to live for. Though he finds love again, the calls for his help keep coming.