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.

Just as the dust from the case of the White Bulldog begins to settle in the small Russian town of Zavolzhsk, its sleepy rural existence is shaken up once again by the arrival of a desperately frightened monk who seeks the help of the bishop, Mitrofanii.

The monks have been troubled by visions of a dark, hooded figure that appears to walk on the waters of the vast Blue Lake surrounding their monastery. Sceptical of ghost stories, Mitrofanii sends first his clever young ward, then two of his most trusted advisors, to investigate the mystery. All 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 search of answers. As she delves deeper into the layers of secrecy that cloak the islanders, 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...