NOTE: This spoiler was submitted by Jeremy

The film starts with a woman named Laura (Alicia Silverstone) taking her children Aiden (Jaeden Martell) and Mia (Lia McHugh) to visit their father Richard (Richard Armitage). Upon arriving, Richard informs Laura that he and his new girlfriend Grace (Riley Keough) are engaged and due to be married in the fall. Laura storms out and returns home where she commits suicide by blowing her brains out.

Laura is laid to rest by her family. All are distraught, but Mia is inconsolable since, being a devout Catholic family, she believes Laura won’t get into heaven for killing herself.

Six months later, Richard tells Aiden and Mia that they are planning to spend Christmas at Grace’s family’s lodge in Massachusetts in an attempt to bond. The kids are both heavily against this idea since they blame Grace for their mother’s suicide. Aiden curses at Richard for picking a “psycho” over Laura.

Aiden and Mia go on Richard’s computer to look up an article about Grace’s past. She was part of a religious cult led by her father, and everyone but her committed suicide so that Grace can live and pass on the cult’s teachings. Photos are seen with the cult members blindfolded and having a piece of tape on their mouths with “SIN” written on it. Although Richard does know about this, he and Grace want them to understand that she has put this behind her and has tried to adjust to a normal life.

The kids join Richard and Grace on their trip to the lodge, along with Grace’s dog Grady. Grace makes an earnest attempt to connect with Aiden and Mia, though they are cold toward her. Upon arriving, everyone gets settled, with Grace and Richard fooling around in the next room, which the kids can hear.

Everyone goes outside on the icy lake. The kids get upset when Grace comes out wearing a hat that belonged to Laura. Moments later, Mia drops a doll in the water and tries to reach it. Grace gets her away and tries to get the doll herself, but the ice breaks and Grace falls through. Richard manages to pull her out before long.

Richard is called to work in the city until he can return for Christmas, leaving Grace in charge of the kids for the time being. He leaves her with his gun just in case. Grace keeps trying to befriend Aiden and Mia, but they continue to shun her or just rudely dismiss her. Grace catches Aiden watching her as she showers, and they make it clear that she will never match up to their mother. However, Grace calls Aiden out for doing this, and slowly gets Mia to warm up to Grace, even watching TV with her.

The next day, Grace finds that some of her belongings, including her medication and food, has gone missing. A storm is going on outside, leaving them snowed in, and the electricity has gone out on everything, including their phones. Grace suspects the kids are pranking her, even going as far as to snap and swear at them. They claim that some of their items have gone missing as well. Around the cabin, Grace sees things like clocks being forwarded to a later date, and words like “REPENT” written on the mirror. Soon, even Grady goes missing.

Days pass, and Grace grows more anxious due to the lack of meds, as well as the increasing cold and hunger. She thinks she hears Mia talking on the phone and barges in to expose her lie, but finds that it is off and that she was just pretending to talk. Other weird things happen, like Grace hearing the voice of her father preaching his sermons. Without her meds, Grace also begins sleepwalking and seeing bizarre visions. Aiden tells her he had a dream that the heater malfunctioned and killed them all, and he thinks they are dead.

Grace attempts to go into town for help, but ends up getting herself lost. She wanders in the cold and barely visible area until she finds a cabin shaped like a cross. She sees what appears to be her father calling to her, but Grace ends up making her way back to the lodge, where she finds a frame with a picture of Aiden and Mia that says “In Loving Memory”, along with a story about them all having died due to carbon monoxide poisoning on December 22nd. Aiden continues to insist to Grace that they are dead. She later comes across him hanging, but he seems unaffected in an attempt to prove that they are in purgatory.

Grace finally loses it and goes out in the snow aimlessly, walking on her knees and praying. She comes back with Grady, who was frozen to death. This causes Mia to break down and cry, as she blames herself for what happened as she left the door open for him to get out. Grace truly believes that they are in purgatory and can only repent for their sins. She stays outside the whole night even as Mia pleads with her to go back inside.

Grace begins her penance by burning her legs on hot logs. The kids begin to grow concerned for Grace, as it is revealed that they were gaslighting her into thinking she was going crazy this whole time. They stole her meds and other belongings and hid them in a crawlspace. They drugged her tea and played recordings of her father’s voice to torment her. They then lock themselves in the attic as they are now terrified of Grace.

In the morning, Richard returns while Grace is frostbitten and fully mad. She has the kids with her and is holding Richard’s gun. Richard tries to talk her down, but she is convinced they are dead, and she puts the gun to her head and pulls the trigger to prove it. Richard tries to get his gun back, but Grace shoots him in the head, killing him. The kids rush to Richard’s body before running outside. Aiden tries to drive them away, but the car is stuck in the snow, and Grace gets them.

Grace sits the kids and Richard’s corpse at the table as she forces them to sing a church hymn. She then puts tape on their mouths with “SIN” written on it, leaving the gun in the middle of the table.

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Siblings Aiden and Mia cope with their mother's suicide after their father Richard announces his engagement to his new girlfriend Grace. As a result, the kids are hostile toward Grace even though she tries to make a genuine connection with them. The kids also learn that Grace used to be part of a religious cult led by her father where every member committed suicide except her.

The family goes with Grace to her family's lodge for the Christmas holiday in an attempt to bond. Richard must go to work in the city and leaves Grace in the care of the children. Over time, Grace experiences strange occurrences, like hearing the voice of her father's sermons, or messages on the mirrors. They are left snowed in after a storm and suffer from the cold and a lack of food. Grace slowly starts to lose her mind.

It's revealed that the kids were tricking her into thinking she was crazy, only for her to actually go crazy. After performing a penance since she thinks they are in purgatory, Grace gets Richard's gun and tries to prove they are all dead. She ends up shooting and killing Richard for real before holding the kids hostage, with the implication that she will leave them to the same fate as her fellow cult members.