Heretic: the ending of Hugh Grant's disturbing horror explained

The story follows two Mormon women, Sister Barnes and Sister Paxton, as they spent their days trying to convert people to their faith. They have a list of people who have expressed an interest in converting, with Mr Reed being the last on their list. 

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Visiting his house, he promises that his wife is baking a pie in the kitchen. The trio discuss Mormonism and theology - with Sister Paxton claiming that when she dies, she would like to be a butterfly and rest on the fingers of the people she loves to let them know she's there.

The women become increasingly uncomfortable by the lack of another woman present in the room, which they requested before they came into the house. While Mr Reed goes to 'check on his wife and the pie', the girls spot that the candle that has been burning blueberry pie smell, and that they can't escape through the front door, which has been deadlocked. 

They politely ask Mr Reed if they can leave, and - after a long explanation about his obsession with theology and his disappointment in every religion - except the "one true religion" that he has discovered - he gives them a choice to leave through the back of his house by choosing between two doors labelled 'belief' and 'disbelief'.

Deciding to stick to their beliefs despite Mr Reed's determination to dissuade them of their faith, they try to leave through the 'Belief' door, only to find that both doors simply lead to a basement. While trapped down there, a woman who Mr Reed claims to be a prophet, eats a pie laced with poison and dies. He promises the women that she will be resurrected - and that they will witness a miracle. 

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While waiting, the women's minister arrives at the house to track them down after they failed to return to the church, which they use as a distraction to try and save themselves. While distracted, they realise that the 'prophet' has risen from the dead, and gives them words from the other side, including telling them that "this is not real".

After Mr Reed claims to have shown them a miracle, Sister Barnes aggressively rebukes him, comparing the prophet's claims of an afterlife to a near-death experience that she had as a child. In a shock moment, Mr Reed slits her throat, seemingly killing her. He finds a metal implant in her arm and tells Sister Paxton that they are in a simulation where there are many fake people, and that she is one of them. 

Sister Barnes points out that it is a contraceptive implant before confronting him herself. She works out that the 'prophet' didn't really return from the dead, but was in fact two women swapping places - and that the woman trying to tell them that it "wasn't real" has gone off-book - and was instead trying to warn them about Mr Reed - and his claims of the simulation are to cover for the comments.

Sister Paxton finds the trapdoor that the women would have used for the deception, and moving further into the basement she discovers a group of caged women - finally telling Mr Reed that she knows that the one religion he has discovered is 'control', and that everything she did throughout the evening followed his plan to lead her to the basement room. 

Paxton then stabs Mr Reed with a letter opener that she had hidden earlier. While trying to escape, Reed catches up with her and stabs her in the stomach. While she accepts her death and begins to pray, with Reed approaching her to kill her once and for all, he is hit on the back of the head and killed by Sister Barnes - who wasn't quite as dead as we thought - but dies after saving her friend as her final act. 

Paxton finally manages to escape the house, and is crawling away, about to make a phone call when a butterfly lands on her hand, throwing back to the earlier moment when she claimed that's what she would do when she died.

Fans of the movie have suggested that the ending is very ambiguous, and that it can be interpreted in several different ways. Firstly, it is simply that Paxton made it out of the house with her faith intact. Another is that her faith is confirmed by seeing the butterfly, which could be Barnes appearing to her in a form that she already said she planned to return as. 

However, the butterfly immediately disappeared from her finger, which throws back to one of Mr Reed's earlier monologues about reality, where he tells her about the butterfly dream - about a man who dreamed he was a butterfly but woke to realise he was a man - and that this could all be happening in Paxton's mind. 

Another theory is that Paxton died after being stabbed, and is experiencing the same afterlife that the prophet suggested, including an angel (Sister Barnes apparently saving her), white clouds - as she falls into a heavy snow, and a feeling of derealisation - which would explain the butterfly that disappears from her hand.