Bruno is assigned by Walsingham to infiltrate the Babington plot to kill Elizabeth and put Mary on the throne.
It's been a while since I read the previous one in the main series (2016!) so I struggled a bit w/ all the back references in the 1st 1/3 or so. The story had some exciting moments but also some where I thought the author was trying too hard to find contemporary resonances - to the point where I was thinking Bruno couldn't have said that