Deep in British Columbia, at the turn of the 20th century, lies Winteridge: a small village, perched on a lake made famous by the monster that lives within its depths. 12-year-old Iris Sparks lives in Winteridge with her brother, her working-class Welsh mother, Llewellyna, and her blue-blooded father from England. But amid the idyllic, Edwardian setting, there are dangers lurking, and Iris watches in horror as her mother increasingly succumbs to seizures. Llewellyna has visions of disasters yet to come, and Iris's mind wills these visions to life, blending Welsh myths with Indigenous stories about what haunts the lake.