Nick Stone is an agent with the British Secret Service and the CIA.
He is a versatile fighting machine who has worked for several organizations during his career. Soon after leaving a very unhappy and probably abusive home life, Stone joined the military and was eventually accepted into the highly elite Special Air Service (SAS) where he distinguished himself. He demonstrated regularly that he was smart enough and good enough to get the job done, no matter how dirty it was.
As the series starts, Stone has been out of the SAS for around four years and was working for the British Secret Service, involved in numerous "deniable" missions. He had a few successes and a few less so. His relationship with his bosses is less than perfect as their covert, often double-dealing methods fly against his military training and bearing, but he went where he was told and got that job done as well.
During the series progression, he meets and falls in love which causes him to seek American citizenship. This is offered but with the proviso that he now work for the CIA, thus part way through the series he changes employers. The type of work remained the same. Stone is a skilled operative who could do any kind of mission but his ability to kill quickly and dispassionately often dictate the kind of task he is given. Just because his bosses want him to kill, though, does not mean that Stone will actually do it.
Being a man with a conscience, Stone's various missions weigh heavily on him and he constantly desires to get away from the killing and the pain. When you are as good at it as he is, though, it's hard for others to stop calling.