On the isolated island of Brack, the people live by an ancient bargain: every year, a sacrifice must be made to the Glimm, the creature that haunts the salt marshes.

Eight years ago, it should have taken Lotta. Instead, the monster spared her and claimed her pony, shattering tradition and anointing her both folk hero and object of suspicion.

Now sixteen, Lotta tends the Council's sacrificial horses, keeping her distance from the islanders who whisper about her fate.

But something is stirring. The island hums beneath her feet, and a song threads through her dreams. Is the Glimm calling Lotta back?

When she crosses paths with Moss - a boy once hidden from sacrifice - a daring bog rescue sparks a chain of events that forces both of them to question the Council's rule and the lore that binds their people.

As crops and animals are blighted, and uncanny weather plagues the community, Lotta and Moss must decide whether to obey the Council or risk everything to uncover the island's deepest secrets.

Because on Brack, monsters come in many forms.