Nikki Sinclair is an agent with MI6.
There are two points of extra special importance concerning her that need to be mentioned right up front to have a better understanding of her and her adventures with the Secret Intelligence Service.
One is that the year her activities begin is 1974. The Cold War is still very hot and the need for qualified agents able to speak more than one language of those in Warsaw Pact is high. Sinclair's skills in this regard are more than impressive and her intelligence also a major plus. Toss in the fact that she is an extremely attractive and desirable woman in excellent physical condition and you have the makings of a highly useful operative.
The other is that she is a lesbian and in the climate of the 70's, homosexuality is not only very much a social taboo, it is considered an illegal activity in most places Sinclair might be sent. Even worse from an Intelligence point of view, her sexual persuasion is considered by her bosses to be a major security risk making her open to blackmail on one side and seduction on the other.
Those two factors are key elements presented to the reader early on in the introduction to Sinclair when she is nearing her graduation from university. One of her professors is a talent scout for MI6 and alerts his friend, Ponsonby-Davies, of the excellent candidate he found in her. "She's a damn fine linguist. And she's beautiful enough to turn any man's head when she enters a room. Also she keeps calm under pressure, has no friends to speak of and appears to be estranged from her family. You won't find anyone closer to filling your requirements than [her]," he tells PD, as the MI6 man is known by.
PD will concur and will bring her into the business under his tutelage and control. He will prove an interesting and often frightening handler. A mentor at times and a Svengali even more often, he will send Sinclair into numerous situations where she will have to use all her skills, instincts, and chutzpah to survive.