The episode begins with Mona creepily painting a mannequin head while she talks to A about a plan. It's Halloween and the girls talk about going to a party that night on a train. They'll be wearing surprise costumes from movies, as a part of the theme. Hanna's mother on the other hand is getting serious about Pastor Ted, from the look of things. A house they pass has a casket with Alison's name on it. Emily walks up to move it and the kid who lives there jumps out and scares her. Spencer is surprised to see Garrett in her house dropping something off for her sister. He wants closure he explains to her and she in turn asks him what he knows.
Before he can tell her anything Toby walks in and tells him to leave. Garrett does as is told. Toby then justifies his actions to Spencer by saying, he doesn't want to leave her alone with him as he feels it’s not safe, and hence asked Garrett to leave. Hanna and Caleb are still going strong as it turns out and apparently still keeping things secret from each other. He won't be attending the party, he tells her. Ezra too tells Aria he can't go to the party, much to her dismay. He says it is something work related, because of which he shall have to give the party a miss. The girls start arriving at the party. Hanna and Aria arrive early and end up running into Jenna and Noel. Spencer and Toby arrive. Noel throws up a nut and catches it in his mouth, then begins choking.
Turns out, it was just a prank that ends up freaking out Spencer. Emily and Paige arrive. Jason walks in and the girls are surprised to see him back in town. Ashley and Ted go all-out to entertain the kids who drop by. Mona sneaks out of her hospital room, leaving the mannequin head on her pillow. Somebody with a mask grabs Hanna on the dance floor. She's not happy, from the looks of it. Spencer runs into Jason. He says he was going to stop by but didn't when he saw she was talking to Garrett. He doesn't think Garrett is innocent, and makes this fact amply clear. A sad-looking girl tells Ashley she's scared and wants to call her mother. Ashley questions her about it. She makes a strange comment about scary things not being fun, which Ashley finds baffling. It turns out the masked stranger who grabbed Hanna was Caleb.
The two kiss, as Hanna apparently is happy to see him there. Aria starts talking to the lead singer of the band. While she is at it, somebody with an elaborate mask walks by Aria's table and drops a pill into her drink. Paige and Emily seem very happy to be with one another. Somebody is then shown spying on them, without them being any the wiser. Spencer is grabbed by somebody in a mask too. It turns out to be Garrett, who apologizes for startling her. He says he's going to tell her everything about the night Alison was killed and then he's leaving town forever, as he is done with this town and wants to start afresh. He says that night he took Jenna outside to let Ian and Melissa talk. He says they went behind the DiLaurentis house, which leads to a flashback. That night Garrett and Jenna run into Alison.
Alison and Jenna start pushing and shoving, with Jenna taking a tumble. She wants Garrett to do something. He pretends to hit her with the shovel and the then-blind Jenna asks if Alison is dead. Garrett grabs her and they leave. Garrett tells Spencer he didn't kill Alison, but Jenna thinks he did. Garrett returns later to see Alison talking to Byron. She appears to be threatening him. In present day Spencer wants Garrett to tell Aria what he just told her. She tells him to wait while she goes to get her, so he can in detail describe everything to her too. Spencer and Hanna see Aria's purse, but there is no sign of her. They both get a text from A which reads "Guess who won't make it to the end of the line?" Hanna thinks Garrett could be messing with them. Spencer goes to look for Aria and the other two plan to notify their significant other that, A is probably on the train.
Next, Aria's is shown hidden inside a boxy someplace with tape over her mouth. Somebody grabs Hanna wearing a mask similar to Caleb's. But when she spots Caleb across the room she turns around and pulls it off, in order to unmask the intruder. The person was wearing another mask underneath, as it turns out. Ashley can't find the blonde girl from earlier, try as she might. Ted never saw her. Ashley eventually finds the girls upstairs standing in one of the rooms. She says her mother got upset when she called, then tells Ashley about her sister telling lies about her. She feels cold and Ashley puts a blanket over her. Ashley goes downstairs to get Ted, but when they come upstairs the girls is gone and the blanket is folded neatly on the bed, much to her shock and horror.
The person who drugged Aria grabs Spencer and starts choking her. Spencer manages to get away for a bit after a struggle, but the person grabs her again. Paige eventually comes to her rescue. A piece of the person’s costume is stuck in Spencer's hair. Aria turns and finds herself, staring at Garrett's lifeless face, which shocks her to no end. Spencer and Hanna discuss the fact A clearly knew she and Caleb had been sneaking around, since the person wore the same costume. In other words, this person knows all the intimate details of their day-to-day life. Paige asks Jenna if she's seen Aria. She is no help, much to Paige’s dismay. The girls find Aria's necklace and Hanna breaks through a locked window. The get to the box, moments before he falls off of the train. They pull her out and find Garrett's dead body, much to their shock and horror.
Ashley tells Ted a lot of things about the girls don't make any sense. She is thinking it might have been a ghost, from the way things have played out. Police arrive at the train and start taking statements, of everyone present at the scene. The girls think the reason A pushed them to get Garrett out of jail was so he/she could kill him, and now thanks to them has actually managed to do so.
Ezra shows up at the scene just then, saying he had been at the end of the line waiting for her and heard from police what happened. Caleb and Noel start pushing each other and the drinks cooler gets knocked over. Inside the ice is a body bag. It is then shown that, Mona has one of the masks A used underneath her bed. She had been on the train, as it turns out. Next, a hand is shown reaching up from the ground somewhere. The episode ends at this point.