Deep in British Columbia, at the turn of the 20th century, lies Winteridge: a small village perched on an enormous lake made famous by the monster said to haunt its depths.

Twelve-year-old Iris Sparks lives in Winteridge with her brother; her working-class Welsh mother, Llewelyna; and her blue-blooded father from England, a progressive bohemian who has brought his family to Canada for an adventure.

But amid the idyllic, Edwardian setting, there are dangers lurking. A blend of Welsh and Indigenous stories of a predatory lake monster take real shape for young Iris as she begins to unravel the truth behind her mother's dark fairy tales, and watches in horror as her mother increasingly succumbs to seizures.

As the First World War reaches its height, Iris must contend with the demands of a deteriorating mother and the harsh realities of a toxic love triangle. All the while, Iris's mind continues to exert its strange and awesome power, and she and her entire community must find a way to survive at the mercy of otherworldly beasts and a hungry darkness.