William Constable is a physician - and an astrologer.
That is a very unusual combination today, or at least I would imagine few doctors of medicine would endeavor to see both profession placed on their shingle outside their place of business. But the period of time Constable plies his trade(s), the middle of the second half of the 1570s, apparently that was not such a peculiarity.
The one thing that Constable would never willingly add to that list of vocations is 'spy' so when we watch him be pushed into that very type of work, it is important to tack on the adjective of 'reluctant'. He gets involved in the machinations and deceits of the Private Secretary to Her Majesty, Queen Elizabeth (the I, obviously), Sir Francis Walsingham, only because that man, who also functioned as an unofficial but still powerful spymaster, called for him and if so directed, nobody in England could afford to refuse.
Constable is a former pupil and then assistant to the noted (at the time) astrologer Doctor John Dee who gained much favor with the monarch for a time (always for a time with her) and learned his star-gazing-and-predicting skills from him a decade before we first meet Constable but apparently they had a falling out and Constable is on his own when a Captain in the Guard comes knocking on Constable's door to take him to see Walsingham. Constable was not in the calmest state of mind when so summoned.
I should add to Constable's lists of expertises mathematics for that is what he started with and is his greatest love. It apparently did not pay the bills that well so he added astrologer to the mix long ago and then a bit later added medicine. "My first love is the subtle mathematics of the heavenly bodies, but have set this aside in favour of the clamour for healing. A reading of the stars is undoubtedly beneficial to treatments, and I possess sufficient vanity to suppose that my ministrations will be more effective than many other physicians I know."
Constable's initial use to the royal spymaster is that of an expert needed to examine and comment on an item that Walshingham's agents had obtained - the previous owner no longer breathing so no longer needing it. But that is just the first step for the examination will lead to yet something else to help the powerful man, an additional piece of investigation that will require far more subterfuge and guile than the good doctor was expecting. Subsequently, Constable would learn that once you proved useful to a man like Walsingham, he never ever forgot you, especially when he needed something else.