After a summer spent in a haunted castle—a summer in which she traveled through time to solve a murder mystery—Kat is looking forward to a totally normal senior year at McTernan Academy. Then the ghost of a little girl appears and begs Kat for help, and more unquiet apparitions follow. All of them are terrified by the Dark One, and it soon becomes clear that that this evil force wants Kat dead.

Searching for help, Kat leaves school for the ancestral home she’s only just discovered. Her friend Evan, whose family is joined to her own by an arcane history, accompanies her. With the assistance of her eccentric great aunts and a loyal family ghost, Kat soon learns that she and Evan can only fix the present by traveling into the past.

As Kat and Evan make their way through nineteenth-century Vienna, the Dark One stalks them, and Kat must decide what she’s willing to sacrifice to save a ghost.