Plot summary
Aria, Hanna, Spencer, and Emily are headed to jail when their van comes to an abrupt stop. 'A' knocks out the driver and gasses the liars, rendering them unconscious. They wake up in their beds, but this isn't home. Frames are missing their photos, windows open to walls of dirt, and cameras watch from the ceilings. The girls reconvene in a shared hallway where a disembodied voice directs them to a re-creation of Alison's living room. Sitting at the piano is Mona Vanderwaal, wearing yellow and claiming to be Alison.
She brings the girls to a child's playroom where they receive invitations to "prom." Hanna tries to incite a prison break, and a loud siren begins to wail. It doesn't quiet until everyone returns to their rooms. Peter and Veronica seek answers from Alison, who tells them that 'A' is back. Without police support, The Hastings work with Caleb, Toby, and Ezra to conduct an independent search. Andrew remotely listens in on their conversations. At night, Mona explains that the dollhouse experiences short power outages on a consistent schedule. A long hallway has a vault on one end and an exit on the other, but neither is accessible within the time limit.
Anyone caught outside of their room will be tortured as punishment. Spencer mentally rearranges the blocks from the playroom to spell "Charles." 'A' gives Mona a gas mask, but doesn't explain why. The girls are forced to decorate for "prom," and Spencer and Mona are strangely enthusiastic. They rattle off a list of materials, secretly planning to build a device that will shut off the building's power. They disguise the finished product as a camera for Aria. Caleb offers Detective Tanner his expertise in exchange for access to the PD's resources.
He locates the missing van at Campbell Farm where the police find 'A's lair. They see footage of the girls, including Mona, in the dollhouse. The girls attend "prom" in dresses provided to them by 'A'. Spencer lures "Charles" out of the shadows, Aria triggers the power outage, and the girls run. Spencer goes to 'A's vault and finds a collection of childhood memories, including a video of Mrs. DiLaurentis, two young boys, and a baby girl. 'A' finds Spencer, but when Mona shows up, they disappear. The girls make it outside, only to find themselves surrounded by an electric fence and miles of forest.
Credits
Adapted from Wikipedia contributors, Pretty Little Liars season 5. HTML formatting and reference markers removed.