Mrs. Merlin is a spymaster.
The time period for the adventures of this formidable former covert operative and now renowned trainer of new female agents is the Napoleonic Age; a decade or so in which every country in Europe which had not already fallen to the control of the French Emperor was doing everything it would to amount a defense against his military genius and his personal appetite for control.
To label Mrs. Merlin as a spymaster is a bit of misdirection though she does play a major role in selecting the assignments the graduates of her school, and even senior active students still learning, are given on behalf of British Intelligence and British Crown. The name of her institute is Mrs. Merlin's Academy for Select Mrs. Merlin is described as "a frail, feather-thin widow with a cap of dove grey curls framing her narrow face. Age had softened her features and blunted the poke of her prominent nose, but behind the oval spectacles, her silvery eyes gleamed with a hawkish intensity". She may be well past her prime but she is totally sharp when it comes to ensuring her students are taught all they need to know to survive in the clandestine world she prepared them for.
Mrs. Merlin tells the Marquess of Lynsley, her close friend and chief governmental confidant, the Academy exists "because there are no easy or pretty answers to the threats our country faces in times of war". She makes sure her girls know this.
The three recorded adventures we have all deal in one level or another with the actions of Mrs. Merlin but for the most part they center on the activities on one particular student - a different one for each mission. While the person might change from one assignment to another, each of the three, in fact all of the young women who have been uniquely chosen to attend the Academy share many aspects.
First, each has been given a new name on entering its world, one chosen at random from the ornate globe in the headmistress's office, said name taken from some city or state or country in the world. A new name for a new life. Moreover, each was given "a new meaning for the savage skills learned in the primitive school of the slums". The girls who receive their lessons here all come from extremely bad situations. "All of them were orphans who had been left to fend for themselves in the slums of London. Alone and homeless, they had quickly learned the cardinal rule of the rookeries - only the strongest survived".
Those lucky enough to have lived long enough to be noticed by the scouts who patrol the back alleys and streets of the bleaker areas of London as invited to join the ranks of Mrs. Merlin. All of them fully aware that the Academy is not "an ordinary institution of higher education" which was not a problem for any of them because "nobody here at the Academy is remotely respectable", as one of the girls points out.
There are truly "standard" school lessons given; each girl was expected to be able to read and write without issue and to have a decent understanding of math and some science as well as learn etiquette and manners and how to behave in polite society. But after that the Academy was quite different. As one student points out, "How many schools for females include a Negro boxer, a convicted cardsharp, and a former courtesan to King Carlos of Spain among its instructors?" ... "Not to mention the Indian fakir". Fencing was a required course, though not really to be able to compete but to actually be able to dispatch an opponent, permanently.
And just as the need to end a target's life might become necessary, so might the requirement to make love, not war. Nothing is out of line for these girls to accomplish their missions. "There are no lines, Sofia," one student is told, "Not here, not in our world. We'll do whatever they ask of us, whether it be slitting a man's throat or seducing the Prince of Darkness."