Bohemia, 1878. Lady Darina Strazova is a brilliant practical chemist in Vienna, forced to hide her scientific discoveries behind her husband’s name. But when she’s caught in the middle of a scandal, she is exiled to her parent’s house in Prague, possibly never to return. Rescue comes in the form of a letter from Darina’s eccentric and ailing aunt, Eva, countess of the famed Houska Castle, who begs Darina to bring her expertise on an urgent and mysterious matter. Darina happily accepts. After all, the old legends about the castle – that its inward-facing fortifications were built to guard against a gateway to hell – have never been more than folklore, tales meant to frighten children. On arrival, she realizes things have changed at the castle. Countess Eva’s health has declined rapidly. There have been sinister disappearances in the local village, surrounded by whispered tales of spirits and monsters. And the many scholars and visitors who once visited the castle’s magnificent library have now been reduced to a single man: Father Conn, exiled to act as the castle’s chaplain after a scandal of his own. Before long, Darina is having visions of an apocalyptic future, and Conn is beset by visitations from a black-robed figure, whose voice heralds terrible, irresistible things. Fighting for their sanity, Darina and Father Conn must uncover the mysteries at the heart of the castle – before its dark promises unravel them in turn.