Following the reveal of a bloodied Clark Hudson tied upon a tree with rope, Emma Duval sends Piper Shaw to the police department while Emma calls Kieran Wilcox. Maggie Duval investigates the death of a police officer handcuffed to a cell and while doing so, she finds a carving of a heart inside his throat. The carving was one of many gifts that Brandon James sent to her two decades earlier.
When Maggie and Emma attempt to get Clark untied from the tree, Maggie accidentally leaks out Clark's blood, killing him.
Meanwhile, at Brooke Maddox's Halloween party, party attendee Grayson Pfieffer is mysteriously murdered in the washroom. Everyone escapes in fear except for Audrey Jensen, who went to investigate the death. When Audrey finds Grayson's body the killer appears and grabs her. When Audrey asks the killer "What are you waiting for?" the killer apparently stabs her in the neck. Maddox returns with two wine bottles upon realizing everyone was gone.
Seth Branson, now escaped attempts to get in the Maddox household, but is unable to due to Brooke locking the doors. When he asks Brooke to let him in, Brooke refuses believing that he is the killer, despite his attempts to explain that he was framed. A brief blackout suddenly appears and when the power comes back on, Branson is gone and The Killer is there. The Killer breaks into the house and gives chase to Brooke who tries to hide in a freezer but The Killer sees her shut the door. The Killer locks her inside and tries to stab her through the doors of the freezer but is only able to inflict minor injuries. Unable to kill Brooke, The Killer turns on the freeze and leaves Brooke to freeze to death inside.
Emma and Noah Foster arrive at the Brooke household and find Piper's glasses and phone in a puddle of blood then assuming the killer had beaten her to death with a car door. Kieran, armed with his gun, arrives only to be saddened by his father's death. The trio unlock Brooke from the freezer and manage to find Audrey, whom was only slashed on the shoulder. Later, The Killer tells Emma that she has kidnapped her mother. In order to get her back, she must meet him at Wren Lake, alone, threatening to kill Maggie and more people if she does not comply. Emma does so and at the lake, Emma finds Maggie tied up in a chair at the docks and rushes to her. But before she can free Maggie, the killer appears and Emma demands to know his identity. Seeing no further reason to conceal the truth anymore, The Killer slowly reaches up for the mask and removes it.
The Killer is finally revealed to have been always been Piper Shaw (somehow, she faked her death), shocking both Emma and Maggie. Piper explains that she had been behind all of this from day one. She reveals that her real father is Brandon James; she was the baby that Maggie had sent away for adoption and never gotten to know. Upon finding out about this, she has always viewed Maggie as the true "monster" for her part in getting Brandon killed. She was also jealous of how she was sent away for adoption while Emma had the perfect life. She vowed to make them both pay for it all. Emma asks why she had killed her friends since they didn't have anything to do with Brandon's death. She replies that all that was meant to do was make Emma suffer which she had enjoyed greatly, disgusting Emma.
Piper goes on to explain that Branson is tied up in the trunk of her car, whom she intends to frame for the murders. Piper continues to say that she will make up a story that Branson had killed Emma and Maggie but not before Emma fatally stabs Branson. Eventually, Emma gets angered enough to attempt to attack Piper with a knife she had hidden behind her back, only to be viciously slashed across the stomach (Piper knew she had the knife all along), critically injuring her. Piper continues to rant on how Emma is so "predictable" and that she will kill Emma in front of Maggie and then leave Maggie to die after stabbing her. Maggie manages to free herself from her binds due to Emma's efforts and tackle Piper to the floor but is in turn stabbed in the stomach. Piper then goes on to attack Emma, repeatedly attempting to slash her some more. Soon enough, Piper injuries Emma further and holds her at knifepoint.
Piper tells Emma that she has one more surprise to share and that she'll "never see it coming" but before she can say what it is, she is shot in the chest by Audrey with Kieran's gun, causing Piper to fall into the lake. Audrey explains that she had gotten her hands on the gun when Kieran handed it to Noah but Audrey took it from him as she knew how to use it. She saw Emma running to the woods and followed her she then found them all at the dock. She then comments "bitch talks too much". Piper survives and begins to climb back up the dock for one last attempt to kill them. Emma notices this and she shoots Piper in the head before she can attack, finally killing her and ending the killings.
The following morning, Maggie is sent to the hospital while it is revealed to the world that Shaw was the murderer and Audrey saved Emma from near death. Audrey had letters from Shaw talking about something, implying that Audrey was killing people with her (likely the secret that Piper tried to tell Emma). However, she burns them, now redeeming herself and covering up her "involvement".