Short version:
Natalie and Brooke are the only survivors. They manage to stab the killer in one of Hell Fest’s haunted houses and leave him for dead; however, when police run in to investigate, they can find no body. We then see the killer arrive home to his young daughter, who greets him enthusiastically.

Longer version:
Natalie returns from college and attends Hell Fest (a traveling Halloween-themed park) with her best friend Brooke, Brooke's new roommate Taylor, Gavin (whom Natalie had been flirting heavily with the last time she was home), Brooke’s boyfriend Quinn, and Taylor’s boyfriend Asher. Natalie is nervous—even moreso when Taylor tells her about a girl who was murdered at a previous stop on Hell Fest’s tour (which we saw in the opening scene). The backstory is never explained but as Natalie hangs out with her friends it becomes clear that she has been very distant and has been going through vague hard times that have kept her away.

A man enters Hell Fest (he is only shown from the back so his face is never shown). He sees a girl telling a costumed employee that he isn’t very scary. The man pulls a mask from his pocket, puts it on, and pulls up his hood. He then steals a knife from a food vendor and begins to follow the girl.

Natalie, Taylor, and Brooke run into that girl in a haunted house. She is running frantically and begging for help, but they assume it’s part of the show. The girl hides just as the masked man comes in and stares them down, apparently angered by Natalie’s comment that he doesn’t look scary. Unnerved, Natalie decides to rush the “show” along and tells him where the girl is hiding. The masked man pulls the girl out and stabs her as the other girls leave. Natalie wonders if what they just saw was real when a costumed employee seemingly stabs a girl right in front of them, then reveals it was a retractable knife and everything is fine. As they leave to catch up with the boys Natalie notices the masked man standing behind them, watching.

From there, the masked man continues stalking Natalie and her friends—spooking them, stealing Natalie's pictures from a photo booth, etc.--and kills them in this order:

  1. Gavin, who says he’ll meet up with the others later so he can go back to the arcade area to try to win Natalie a stuffed animal. The masked man gets him alone and kills Gavin on one of those carnival games you hit with a sledgehammer to ring a bell.
  2. Asher, who is separated from the others in a haunted house and gets stabbed in the eye with a prop syringe.
  3. Taylor, who nearly gets her head chopped off in a guillotine after volunteering to be in a show but escapes only to get stabbed in front of other park-goers who think it’s part of the show.
  4. Quinn, who tries to save Taylor only to get stabbed himself—letting the other park-goers know that something is horribly wrong.

In the chaos following Taylor and Quinn’s murders, Brooke and Natalie inexplicably run into a haunted house they somehow mistake for an exit. Natalie realizes there are motion sensors in each room that set off the “traps,” which gives away their location to the killer. They avoid the sensors and find a closet to hide in, then wait for the killer to go by so they can sneak back toward the entrance. Unfortunately, the killer locked the door behind them so they have no choice but to go back through the haunted house and toward the killer. They are almost caught but disguise themselves as mannequins to get the element of surprise. In the ensuing fight, Natalie tells Brooke to run for the exit before getting knocked out. Brooke is wounded in her leg and has a hard time running away, so it doesn’t take long for the killer to catch up to her. Just as it looks like curtains for Brooke, Natalie pops out of a secret door and stabs the killer. They both run to the exit.

Police run into the haunted house but only find a bloodstain on the floor where the killer had last been seen.

We then see the killer arriving home from behind (so once again we never see his face). He places the mask and the photo booth pictures he had stolen from Natalie in a locked cabinet with a collection of masks and souvenirs from other kills. He enters the house and finds a little girl asleep on the couch. She wakes up and sees him, then excitedly runs to him, calls him daddy, and asks if he brought her anything. The man gives her the stuffed animal he took from Gavin just before killing him.

Thanks Greg B!