While diving in a remote French lake, a couple of YouTubers who specialise in underwater exploration videos discover a house submerged in the deep waters. What was initially a unique finding soon turns into a nightmare when they discover that the house was the scene of atrocious crimes. Trapped, with their oxygen reserves falling dangerously, they realise the worst is yet to come: they are not alone in the house.

Scarred for life after a harrowing near-death experience, emotionally fragile mother-to-be Sarah is still struggling to come to terms with her loss. Overcome with silent grief, Sarah now seeks solace in work; however, she is on a collision course with sheer terror when a knock at the door in the dead of night chills her bones to the marrow. Now, the raven-haired, late-night visitor in black wants something precious from Sarah, and will stop at nothing to get it.