This is a very British police procedural. Set in 1980, our heroine, Jane Tennison, has managed a transfer to the Flying Squad. She has only ever wanted to be a police person and she sees this as promotion based on merit. Actually, like a lot of female advancement in the 80s, it was to see if women actually could do the previously only male job or if her failure could be used to deny other women the opportunity.
A side word, if you have read many British police thrillers, you know something about the differences between British and American police structure. In England, armed robberies are investigated by the Flying Squad. They do not investigate other crimes and other police do not investigate armed robberies. The same villain may be implicated in different crimes, and be investigated by different departments of the police. I think a big part of the difference here is the difference in gun culture. Armed robbers will have weapons. Other criminals don't generally use them in the commission of their crimes. Remember, this book is set in 1980.
Jane walks into an armed robbery on her first day. Her superiors in the squad don't want her there. They don't believe she's up to the danger and actively hinder her work. Even so, Jane turns up the best descriptions of the robbers. She goes on to develop the best leads and ultimately the evidence needed to send the gang to prison. A little far fetched, maybe, but as part of the successful woman subtext, very impactful. It was fun to return to the time where woman were mostly patted on the head and then ignored. I enjoyed the book.