Mona is released from Radley and returns to Rosewood High. She isn't exactly welcomed back into Rosewood with open arms. She posts a video apologizing for her past sins, winning over some of the students. The Liars, of course, still think she's up to no good. Mona's not the only familiar face that Aria, Spencer, Emily and Spencer see at school. Aria is shocked to discover Meredith is now substitute teaching in one of her classes.
Meanwhile, Spencer wants Aria to confront Byron about what Garrett said the night he was murdered. Emily's parents are being extremely protective due to everything that's happened recently. Spencer doesn't suspect anything is different with Toby. Hanna tries to find out what's going on with Lucas. And Aria still hasn't told Ezra about his son.
The Liars spot Harold, from the Lost Woods Resort, in the halls at Rosewood High, and they wonder if he has a connection with Mona. During a 10K Charity Fundraiser at the school, they use the run as a diversion and break into Harold's office in the school basement. They find Alison's journal and learn that Ali blackmailed Byron over his affair with Meredith before she died!
Speaking of Meredith, during the 10K, there is an explosion and she is injured. Byron and Meredith think the Liars are responsible after finding a suspicious note, and Byron confronts Aria about the note. Aria tells the girls that "A" must be behind it.