After two years and 47 episodes, fans of ABC Family's Pretty Little Liars will finally learn the identity of the mysterious "A" character who's been tormenting Hanna Marin (Ashley Benson), Spencer Hastings (Troian Bellisario), Emily Fields (Shay Mitchell) and Aria Montgomery (Lucy Hale).

In a sneak peek from Monday's season finale, the girls each receive an invitation to the Rosewood Junior Society Masquerade Ball.

"'A' wants to play hide and seek," Emily says.

Aria receives an extra note from "A" with her invitation, scribbled in red ink: "Be there when the clock strikes midnight."

Though they can't be sure, the four friends believe Spencer's older sister Melissa (Torrey DeVito) could be "A" -- and the masquerade ball will be their chance to confirm or disprove that theory.

"Hide and seek was my favorite game with Melissa. Wanna know why?" Spencer asks. "I always won."

The season finale of Pretty Little Liars airs Monday at 8 p.m. EST on ABC Family.