The Little Liars receive invitations to a masquerade ball along with a message from "A" that demands their attendance. They comb through the contents of Alison's bag once more and find a clue, which leads them to the Lost Woods Resort. They drive out and rent room #1, matching the number on the postcard they found in Ali's things. Once the innkeeper leaves, they steal the guest book and discover Alison checked in twice. Meanwhile, Emily gets a call from Maya, but when she tries to answer, she can't get any reception.
The next day, on her drive home from the resort Spencer sees Toby, who was hired by Jason to clear away debris at the DiLaurentis house after the fire. She tries to talk to him about Jenna, but Toby refuses to acknowledge her. Just after Spencer leaves, Toby receives a call from Dr. Sullivan! And then, Jenna has a mysterious secret meeting with someone at a park! Later, The girls arrive at the masquerade together, where Caleb and Ezra surprise Hanna and Aria. Spencer and Mona talk about the last time Mona saw Ali alive in Brookhaven, and something Mona says makes Spencer realize that the key to room #2 at the Lost Woods Resort was missing. Spencer and Mona leave the masquerade together to drive back to the motel. They sneak into room #2, which is plastered with pictures, news clippings and belongings of Alison's: it's "A's" lair.
While at "A's" lair, Spencer begins to notice the pieces of the puzzle falling into place. When Mona momentarily leaves the room, Spencer finds more clues pointing to "A" being MONA! And when Mona returns, she's wearing "A's" black hoodie and knocks Spencer unconscious!!
Spencer wakes up in the car with Mona driving recklessly. Hanna calls Spencer, who answers the call covertly so that her friends can hear her conversation with Mona. The girls figure out that Mona is driving to Lookout Point, and they race to get there first. Back in Mona's car, Spencer pulls the emergency break, causing the car to skid to a stop long enough to escape. Mona jumps out of the car after her just as other Liars drive up, and Hanna careens towards Mona but at the PLL's screams, slams on the brakes just before she strikes her down. Mona screams, runs and tackles Spencer and tries to push her off Lookout Point, but she slips and falls off the edge herself.
Shortly after, the authorities arrive (along with Dr. Sullivan!), and they discover Mona survived the fall. Dr. Sullivan reveals that "A" blackmailed her by threatening her son, which is why she disappeared, and that it was Toby who convinced her to come back. Toby tells Spencer he had to pretend not to love her, and the couple makes up with a passionate kiss.
Later, the Little Liars head to Emily's house to spend the night when an ambulance rushes past them. The police are waiting, and Emily runs up to her mom, who tells her that the police found a body... and they think it is Maya's. Emily is distraught. Meanwhile, Mona is hospitalized -- and at the end of the episode, someone wearing a red coat visits Mona in her cell. Mona tells the visitor, "I did everything you asked me to."