The Grimsbane women have been hunting witches and monsters for generations—ever since the Watcher, the most powerful witch in the Midwest, cursed the Grimsbane men to die untimely and unusual deaths. Part-time skater, full-time troublemaker Anna Grimsbane may be only twelve, but she's been learning about hunting her whole life and is tired of waiting to do the real thing. She and her twin brother Billy are about to turn thirteen, the age the curse takes hold, and Anna wants to be on the front lines fighting to break it.

Only hours before he'll become an accident-prone walking disaster, Billy runs away to find the Watcher himself. The Grimsbane women are all out on patrol, leaving it up to Anna and her friends Suvi and Rosario to find Billy before his recklessness hastens his demise. But the woods are crawling with cryptids, most of whom hate humans, and all of whom hate the Grimsbanes. And the deeper Anna gets into the forest, the clearer it is that reading about witch hunting is no replacement for practical experience.

Anna feels in over her head, especially as she starts to suspect she knows much less about her family history than she'd thought. As she races against the clock to find Billy before midnight, it becomes all too evident that he isn't the only Grimsbane at risk for a grisly death tonight.