Settled down in his beloved Mortlake once again, philosopher and astronomer John Dee is enjoying the quiet life when Her Majesty's spymaster Sir Francis Walsingham darkens his door, instructing him to befriend a certain Polish count with an interest in alchemy and angelic communication. But when Dee and the count are caught breaking the law, trying to convene with angels, they are exiled, and must make their way to the court of Rudolf, Holy Roman Emperor, in far-off Prague.

Meanwhile, Queen Elizabeth learns of Catherine de Medici's infamous Flying Squadron, a group of seductive ladies-in-waiting scattered throughout Europe acting as her agents, and that one of them is already at work in Prague, bending the Holy Roman Emperor to join France and Spain in a crusade to crush Protestant England. With Dee on his way to Prague, Walsingham sees an opportunity to tip the scales. To convince the famously occult-loving emperor to join his side, Dee must distract him with the esoteric Book of Loagaeth, a volume transcribed from the angels that will, once decoded, allow control of the levers of heaven and earth, and everything in between.

Terrible dangers await Dee in Prague, as he faces challenges at every turn, including a reluctant female spy and a ruthless old enemy, and he is forced to call upon the many unexpected talents of his wife, as well as the playwright Christopher Marlowe.