Summer 1312. The brutal murder of King Edward II's favourite, Peter Gaveston, unleashes a horde of demons...

Sir Hugh Corbett, Keeper of the Secret Seal, hastens to the Dominican Priory at Blackfriars where Gaveston's corpse awaits burial. But, on arrival, Corbett discovers that a series of macabre murders has turned the priory into a mansion of death, and a killer is roaming free.

Meanwhile, rumours spread that the pirate ships of the Black Banner Fleet are intent on entering the River Thames and, if the Sea Beggars succeed in their mission, they will weaken the king's power throughout the city. Once again, Corbett must employ his wit and ingenuity to navigate the dangerous and deadly challenges ahead and bring the culprits to justice before matters turn grave indeed.