Sister Pelagia, bespectacled, freckled, woefully clumsy and possessed of a not very nunnish aptitude for solving crimes, returns in a tale of monastic intrigue, murder and adventure.
The small Russian town of Zavolzhsk is shaken up by the arrival of a stranger: a desperately frightened monk from the island monastery of New Ararat. The monks have been troubled by visions of a dark, hooded figure: a figure that strikes terror into the hearts of all who encounter it.
With sightings of the Black Monk occurring with disturbing frequency, and rumours of suspicious deaths reaching his ears, the Bishop decides to send two of his most trusted advisors to New Ararat, but they too meet with unexpected fates.
Finally, Sister Pelagia takes matters into her own hands, and adopting a number of ingenious disguises, she ventures across the Blue Lake in pursuit of the Black Monk. But as she delves deeper into the layers of secrecy that cloak the island and its strange population, and as the body count continues to rise, Pelagia begins to realise that an encounter with a ghost may be the least of her problems.