Mia Myhr moves with her parents and sister Hanna to her grandmother's house on the edge of a forest. Her grandmother disappeared without a trace five years ago. Mia takes her drawing pad into the woods, where she discovers a pond that has never been there before, and catches a glimpse of a terrifying, mysterious animal. When her sister and friends go missing after a swim to the pond, Mia must try to figure out what has happened - by diving under the dark surface.