Aria, Emily, Hanna and Spencer finally decide it is time to tell someone about "A". Meanwhile, Hanna's infamous grandmother drops into town for Tom's rehearsal dinner.

Radley Sanitarium has given Mona a clean bill of health and she is headed back to the halls of Rosewood High, much to the Liars' dismay. With their former tormentor now back in their everyday life, Aria, Emily, Hanna and Spencer take very different views of the "new" Mona. She made their lives a living hell... Is she really cured? And what other secrets could she be holding onto? It's up to Mona to prove to the Liars if she has changed or not.