PLL, S2-E24: Unmasked

A invites the girls to Rosewood's annual masquerade ball, and threatens to kill one of them if they don't return A's cell phone. They have until midnight the night of the ball to hand over the phone, but the girls resolve to find out who A is before then. Using the postcard from Ali's bag and business pen, they find the Lost Woods Resort and discover that Ali checked in to the hotel the day she disappeared. At the ball, Mona reveals that she saw Alison in Brookhaven spying on someone. Spencer figures out that Ali was spying on A and not vice versa and returns to explore the hotel with Mona. She finds A's secret lair, but also discovers that Mona is A. After surreptitiously answering Aria's phone call, Spencer notifies the girls of the danger. Aria, Emily and Hanna arrive just in time to save her from Mona who ends up tumbling off a cliff. Mona survives her fall and ends up in a mental institution.

Source: TV Fanatic