Five days into the future, Rachel Harkin and Nicky Cunningham's wedding looks like a bloodbath. In the present, as Rachel and Nicky drive to his parents' remote cabin, they find a baby alone in a car at a rest stop. Unable to find the baby's parents, Rachel goes to another stop to call for help while Nicky stays with the baby. At the next stop, a nearly empty dive bar, Rachel is stalked by a man whom she stabs through the hand with her car keys, but he is unperturbed and asks her if she is "sure he's the one". Rachel returns to Nicky in a panic, and he reports that the baby's parents had gone for a walk and attacked him when they found him with their baby. The couple make it to the cabin, where Nicky's sister Portia tells the story of the "Sorry Man", who comes up from hell to cut women open in hopes of finding his wife. Later that night, Rachel encounters Nicky's mother Victoria, who cryptically tells her that they will not be seeing much of each other. Rachel finds the message "DON'T MARRY HIM" on a returned wedding invitation.

Four days before the wedding. Rachel cannot find her wedding dress, though she is sure that they brought it. Nicky offers to drive back to Chicago to see if they left it behind. Portia suggests that Rachel wear Victoria's wedding dress. As they are doing a fitting, they make major alterations to the dress. As Rachel goes on a walk through the woods, she finds her torn-up wedding dress on an effigy hanging from a tree, then also finds Nicky's father, Dr. Boris Cunningham, digging a grave. Nicky arrives back at the house and his parents reveal that Victoria has a major brain tumor and the wedding is being planned as one last hurrah. Jude, the young son of Nicky's older brother Jules, confesses he made the effigy to confuse the Sorry Man.

Three days until the wedding. Portia is trying to make all the finishing touches to the wedding. Rachel has to figure out what everyone wants for dinner at the reception. Nicky confronts his father, as he is working on his taxidermy, to find treatment options for Victoria. Nell and Jules have an altercation in the bathroom where she pushes his head under the water in the bathtub. Rachel brings together everyone to try out some therapy. Using the wedding dress effigy as a proxy for Victoria, they all say what they are feeling about her impending death, and then burn the effigy. Jude, using his new video camera from Rachel, explores the basement, where he records a man standing up against the wall. Rachel gets up in the middle of the night to get a glass of water and is attacked by an unknown assailant.

Jude wakes up in the middle of the night and hears the struggle happening. He goes to his father and shows the video he took of the man in the basement. A home video, from January 13, 1997, shows Rachel's parents eloping to the area that the story is currently taking place at. On that day, they get married in an empty church with one witness, who secretly asks Rachel's mother if she is "sure he's the one". When they get back to their cabin resort, Rachel's pregnant mother, Alexandra, starts bleeding from her eyes and nose before dying, so her husband has to cut the baby Rachel out of the womb. It is revealed in the present day, that Rachel was abducted by the man that Jude recorded, who is her father, and was showing her the home video. He reveals that he was the one that sent her the warning in the mail, but Rachel explains that she never sent him an invite in the first place. Jules then breaks into the, now abandoned, cabin resort, claiming that Rachel's father is the Sorry Man after witnessing him cut out the baby Rachel that fateful night. During the chaos, Rachel escapes unnoticed and heads back to the dive bar where the man, the witness from the wedding, returns the favor and stabs Rachel in the leg with a fork. He then tells a story of his great-great-great grandmother, who made a bargain with Death to revive her deceased fiancé, adamant he was her soulmate, the price being a family curse where any descendant who married someone who was not their soulmate, on the sunset of their wedding day, would suffer a painful haemorrhage until they ultimately bled out. The Witness was going to marry Marianne, Rachel's great-great-great-great grandmother, but did not go through with it, leading her to marry Thomas Harkin, transferring the curse to her bloodline, and as punishment for this, Death would make The Witness immortal and forced to be the witness to every Harkin wedding.

Rachel and Nicky suffer through a wedding rehearsal reception where they are inundated by bad relationship advice and questionable stories about other people's relationships. Nicky's mother tells how she met his father in the ER when she needed to have a finger sewn back on. Rachel is forced to repeat the meet-cute story of how she and Nicky met—she was afraid her plane was going to crash, and Nicky (claiming to be hesitant about the same flight because she would not board) spontaneously offered to drive her, telling her she just saved his life. Jules, obsessed with saving Rachel from dying if Nicky is not her soulmate, challenges the story until he can prove that Nicky was lying about he and Rachel initially being on the same flight, leaving Rachel more unsure than ever. At the dinner that night, she tells them about the curse, including the part Jules did not know—that Nicky's family will die if she does not.

Rachel and Nell search for any of her ancestors that might have survived the curse. They find that only one did not die on her wedding day. Nell notices that all the wedding certificates have the same witness signature. They do find one woman who stayed married for 57 years. Meanwhile, Nicky and Jules join their father for a fox hunt. Jules admits that he and Nell are getting divorced, and their father admits that his marriage to their mother is not what it seems. When Rachel and Nell return, they find that Portia has planned a bachelorette party, and Rachel suggests they do a ritual to summon the dead, receiving the cryptic message "Living Dead". During the ritual, an entity smashes into Portia, breaking her nose. Nell tells Rachel to have faith in the version that loved Nicky for three years, and admits to the divorce as well. Nicky confronts his mother over her infidelity, but she collapses partway through. Rachel and Nell are sharing a passionate kiss when Portia, possessed, interrupts them with the cryptic message "Something living, something dead, something stolen, something red."

An hour before the wedding, Victoria has not woken up after she collapsed. Portia desperately wants to cancel the wedding so Victoria gets the chance to witness it happen, but Rachel does not let her. Rachel reads Portia's book on the occult arts and frantically begins collecting ingredients for a potion that will turn her into Nicky's soulmate. She first collects blood from the Witness ("something red") and a lock of hair from Victoria's head ("something stolen"), convincing Boris not to euthanize Victoria in the process. She then asks Jules and Nell to help her amputate her own right pinky toe ("something dead") before going to collect Nicky's sperm ("something living"). Rachel and Nicky have sex and reconcile before Victoria awakens. As the ceremony is about to begin, Rachel mixes the potion.

Rachel chooses not to drink the potion, putting her faith in Nicky being her soulmate. The wedding goes ahead, but Nicky, shaken by revelations regarding his parents' marriage, unexpectedly says no at the altar. The wedding reception continues as Rachel and Nicky argue in another room, coming to the realization that they are not each other's soulmates. As the sun sets without the couple getting married, the curse transfers to the Cunningham bloodline, and Death (as a screeching, intangible presence who can be felt by the Witness and Rachel), begins to claim all the Cunninghams, who begin haemorrhaging. Jules finds Jude hiding under a table and locks him in a room with a loud TV to protect him from the ongoing bloodbath. Victoria, dazed from the combination of her tumor and the haemorrhage, asks what is happening. Jules blames Nicky, saying none of this would be happening if he hadn’t walked out. Jules explains the curse has spread to their bloodline, and Portia realises she is going to suffer the same fate (due to a marriage in Vegas). Portia begins haemorrhaging, and Death reaches Victoria, who succumbs to the curse. In a desperate attempt to stop the curse, Nicky and his family coerce Rachel into completing the wedding ritual, and the Witness signs his signature. Portia bleeds as it happens. Rachel tells Nicky it isn’t going to work, they both are cursed and they are not soulmates, but Nicky still believes they are. Rachel, feeling betrayed, glances at everyone, and Nicky emotionally asks if they are still soulmates. The curse begins taking effect on Rachel, who stumbles out the altar, and Portia’s condition gets worse. Rachel stumbles as blood starts pouring from her eyes, nose and ears, watching the blood fall on her hands. Rachel stumbles out into the snow, as she falls to her knees, and haemorrhages to death, falling to her back and staining her surroundings with blood. Portia also haemorrhages to death, whimpering as her family cradles her. Death goes over all the Cunninghams as they reel from the deaths of Rachel and Portia, before heading to The Witness, his job finally done. Jules and Nellie are both beside Jude, Nellie turns to Jules and asks why he isn’t cursed, he doesn’t answer, but it is strongly implied that the two are true soulmates. Nellie sheds tears and kisses Jude, and the two are left staring into each other’s eyes. Nicky walks out into the main hall and sees Rachel’s corpse, he thumps on the glass to see if she responds, only to realise she is gone. The Witness has one last meal, before Death finally puts him down. The following morning, Rachel is resurrected, and due to passing the curse to the Cunninghams, is punished by Death to be the new immortal Witness. Nicky cradles a teddy bear, before witnessing the risen Rachel. Bored and jaded, asks Nicky if he knows where her lighter is, due to being in shock from all his trauma and seeing Rachel revived from the dead, does not answer. She walks out the room exasperated, leaving Nicky’s life forever. She finds Jude in the corridor, who is taken aback, but Rachel calms him down, and apologises for everything that happened. She warns him to be extremely careful on who he marries, but reassures that she will be there to witness it. Rachel hugs her nephew goodbye and walks to the dining room. She finds all of the dead bodies from the bloodshed, and the Witness’ body, with a written message, “YOUR TURN!” He leaves behind an envelope of money, a change of clothes, and the key to his truck. She goes to the truck and finds a lighter, and drives off, leaving behind her old life.