PLOT
Two Mormon missionaries, confident Sister Barnes and timid Sister Paxton, arrive at the home of a reclusive Englishman, Mr. Reed, after Barnes locks their bikes to his fence. They enter after being told Reed's wife is preparing a blueberry pie, and they begin to discuss religion, with Reed gradually making several uncomfortable comments about the Mormon faith and the nature of belief. After Reed steps out of the room, Barnes notices that the smell of blueberry pie is from a candle, and the pair realize the front door is locked and they have no phone signal.
Following Reed, they enter his study, where he proceeds to give a threatening lecture on the nature of beliefs, stating that all religions are adaptations of one another, and claims to have found the one true religion. Reed gives the girls a choice of two doors to go through in order to exit the house, one if they still believe in God, and one if they do not. Barnes rebels, refuting several of his claims, before they both enter the "Belief" door, only to discover both doors lead to the same basement dungeon.
A decrepit woman appears, eats a poisoned pie, and dies, with Reed claiming that the pair will witness her be resurrected. A Mormon elder arrives looking for the girls but leaves after discovering nothing, as the girls' bikes are now missing. This distracts the girls, and after returning to the dungeon room, they witness the woman rise again and deliver a description of the afterlife. Barnes rejects The Prophet's description, noting its similarity to textbook hallucinations from near-death experiences. When Barnes gives Paxton the signal to attack, Reed suddenly slashes Barnes's throat and claims she, too, will resurrect.
When Barnes bleeds out, Reed removes a metal object from inside her arm, claiming it is a microchip and using it as proof she was not real, and the world is a simulation. Paxton, however, recognizes the object as a contraceptive implant, and realizes that the entire situation had been orchestrated by Reed; while the girls had been distracted by the Elder's arrival, a second woman appeared and hid The Prophet's corpse, before taking her place and delivering the afterlife description as scripted by Reed, adding an unplanned comment: "It's not real". Reed's killing of Barnes and attempt to convince Paxton of a simulated reality was an improvisation to cover the plan going awry. Paxton discovers an underground chute through which the Prophet's corpse was hidden and continues down with Reed promising it will show her the "one true religion."
Following the path, Paxton finds a chamber full of emaciated women in cages, where she deduces Reed's conclusion: that a desire to control others is the root of all religions. Paxton stabs Reed with the letter opener, but Reed stabs her as she tries to escape. As they bleed in the basement, Paxton begins to pray, claiming to a vexed Reed that it is done to show kindness to others, rather than to produce material results. Reed prepares to finish her off, but a still-living Barnes kills him with a plank of wood before dying. Paxton climbs out of a window and sees a butterfly land on her hand; she had earlier expressed a desire to be reincarnated as a butterfly that appears on the hands of her loved ones. The butterfly suddenly vanishes, leaving an injured and confused Paxton alone in the snowy landscape.