Bea (Rose Leslie) and Paul (Harry Treadaway) are newly married and off to have a romantic honeymoon in a rustic cabin set in a secluded forest. During their stay there, they go to a small restaurant whose owner initially acts hostile and asks them to leave, but later calms down, realizing he was Bea's childhood friend, Will. Will's wife then comes in, acting strangely and saying they need to get away. Afterwards, everything goes normally until their wedded bliss is cut short when Bea goes missing. Paul finds her naked and disoriented in the woods with no evidence as to how she got there or why she was there in the first place. He takes her back to the cabin, where she claims she was sleepwalking due to stress, and tells him to make nothing of it.
As time passes, Bea's behavior becomes increasingly distant and strange. At first, Paul blames Bea's strange behaviour on some encounter with Will, but soon he realizes there's more going on. One night as Paul is lying awake, he sees some bright lights through the windows, but when he goes to investigate he finds nothing. As Paul sees his wife's behavior becoming more erratic he finds unique marks on Bea's inner thighs which she passes off as mosquito bites. Some time later during an argument, Bea locks herself in the bathroom, and when Paul breaks in he finds her repeatedly stabbing herself in the genitals. Paul then ties her to the bed and she asks him to "take it out of her". Paul puts his hand into her vagina and takes out a large worm-like creature. Bea later explains that the night she disappeared into the woods, she saw the same lights Paul had seen and couldn't help walking towards them. She then saw some silhouettes, which apparently impregnated her with the creature. Bea says that she is herself, but they're taking away what's left of her. She says she must protect Paul because they are going to "dispose of him". Bea then knocks Paul out, takes him into the middle of the lake on a boat, ties an anchor to his feet, and throws him to his death while believing that she is protecting him by "hiding" him under the water.
Bea is later shown to be deteriorating, her skin becoming white and flaky. She then meets Will's wife, and they walk away into the lights. The film finishes with Bea saying "Before I was alone, but now I'm not.