At the beginning of the novel, readers learn more about Cora's history: in an attempt to commit suicide following relentless bullying from peers regarding her weight, she found a door into the Trenches, "a magical undersea world where she was a mermaid and a hero, valued for her bulk and her strength".

[1] The theme of heroes and monsters is explored "in many forms here, most notably unpacking the ways that the people in charge always think (or at least claim) that they are doing the right thing, even when it is clearly harmful".

She delivers a plot dense enough for a full-length novel in her signature lyrical prose, exploring the effect of cruel, oppressive systems on children’s psyches, while keeping the series’ fairy tale tone intact".

[2] Kirkus Reviews indicated that "McGuire’s themes [...] won’t surprise readers of this series and her other works, but her usual arguments remain sound, and she tells a good story".

[3] Booklist observed that it "highlights the horror of a world that requires you to deny what you know to be true because it doesn't quite fit", and praised it as "a fantastic and tension-filled addition to the Wayward Children series".