The film begins with two Mormon missionaries, Sister Barnes and Sister Paxton, engaging in a rather unconventional debate about pornography. Their discussion arises when Paxton admits to feeling a twinge of sympathy for an actress featured in a video she watched. With their mission to convert others to their faith, the duo sets out into the local community, eager to spread their beliefs. However, on their way, they encounter a trio of teenage girls who cruelly embarrass Paxton by yanking down her skirt to see if she indeed has “magic underwear.” This incident adds to Paxton's concerns about the ridicule their religion faces, yet Barnes appears unfazed.
As the rain begins to pour heavily, they find refuge at the home of an Englishman named Mr. Reed. Eager to engage, he invites them inside and reassures them that his wife is home – which is crucial for their entry. He mentions that his wife is occupied baking a blueberry pie, a detail that further piques their interest. Once settled, Reed engages them in conversation about various topics including religion, sharing his passionate study of Mormonism and proudly displaying his copy of the Book of Mormon.
Their dialogue takes a serious turn as Barnes reveals the tragic death of her father from ALS, and they adamantly steer clear of discussing Taco Bell. Tensions rise when Reed makes an unsettling remark about her father and the essence of their faith, prompting him to exit the room. Alone, Barnes notices a candle on the table, the source of the delicious blueberry aroma that initially welcomed them.
Feeling increasingly uneasy, the women attempt to leave discreetly but discover that the front door has been locked and their phones are without signal. As they cautiously navigate toward the back of the house, they stumble into Reed's study, where he houses an extensive collection of books. Despite their attempts to politely exit, Reed presents them with a choice between two doors, labeled “belief” and “disbelief.” He then surprises them by pulling out an original version of the Monopoly game instead of something more sinister.
Reed uses this moment to lecture the missionaries about the game's origins with The Landlord’s Game and draws parallels with the song “The Air That I Breathe” by The Hollies, contrasting it with “Creep” by Radiohead, a song he assumes they are well-acquainted with. He positions his argument that all religions are flawed iterations of a single original belief, challenging their own convictions. With resolve, Barnes defends her faith, prompting both women to pass through the doorway marked “belief.”
This seemingly simple choice leads them into an underground dungeon, where an alarming situation unfolds. Reed's voice pierces through the darkness, explaining that they are about to witness a miracle. Just then, an old woman, referred to as the “Prophet” (Elle Young), enters, brandishing a disheveled blueberry pie. Reed chillingly declares that the pie has been poisoned, and that the Prophet will soon be resurrected.
Meanwhile, Elder Kennedy becomes concerned after realizing that Barnes and Paxton have not checked back in after hours. He ventures out to inquire about their whereabouts at homes they intended to visit. When he arrives at Reed’s house, the missionaries try desperately to get his attention by shouting and even attempting to ignite a fire with matches, but Kennedy departs before they can signal him.
Back in the dungeon, the ladies watch as the Prophet seemingly returns from the dead, ominously muttering about the afterlife. Reed descends into the dungeon, boasting about the “miracle,” and offers a chilling proposal: he will sacrifice one of them, only to resurrect her afterward. However, Barnes stands firm, asserting that she understands the truth of the situation. She reveals her traumatic history with an e. coli infection that left her clinically dead, drawing parallels with the Prophet’s description of the afterlife. In a shocking turn of events, Reed violently slashes her throat, claiming she would come back.
Moments later, Reed inspects Barnes’ body and discovers a metal pin, claiming it to be a microchip, asserting that Barnes was never real – a concept that shakes Paxton's reality. Yet Paxton counters Reed, identifying the pin as a contraceptive device, revealing that the initial Prophet had truly perished, leaving only a substitute following Reed's orders. The second woman had simply memorized a script while they endeavored to attract Kennedy’s attention.
Determined, Paxton ventures into the trap door where the dead woman lies, granted entry by Reed who is intent on showcasing his twisted version of the “one true religion.” In a harrowing discovery, Paxton finds dozens of women contained in cages – realizing that Reed's conceptualization of religion is merely a facade for control over others. In a moment of defiance, she stabs Reed in the throat with a concealed letter opener her friend had given her for safety. As Paxton makes a desperate bid for freedom, Reed retaliates and stabs her in the abdomen, and while they both bleed profusely, he bizarrely suggests they pray for their fates.
Paxton gleefully retorts, expressing her belief that prayers are in vain and often serve only as gestures of goodwill. As she begins to pray, Reed, weakened and crawling towards her, prepares for one final strike. However, out of nowhere, he is abruptly struck by a plank of wood studded with nails, dealt by the recovering Barnes, who collapses in Paxton's embrace before fading away.
Finally, as daylight breaks and the storm dissipates, Paxton finds a window and escapes. In a moment of transcendence, a butterfly rests on her hand, fulfilling her earlier wish to return as one in the event of her death. But as she gazes away for merely a heartbeat, the butterfly vanishes, leaving her to reckon with the tumultuous events that transpired.
