Plot summary
Mona is out of Radley and back at Rosewood High. The Liars do not trust Mona. Someone plants a cow brain in Mona's locker kept up with a knife. Mona is appalled and whispers something to Lucas. The Liars worry that people will think they were involved, including Mona, who instantly assures them she believes them, and wants to earn back their trust. When Lucas goes to Hanna's house later to return the rest of the money he owed Caleb, he tells Hanna that Mona had been sneaking out of Radley for a long time.
Emily recognizes the new janitor as the front desk clerk from the Lost Woods Resort. The Liars investigate his office during the fundraiser and find Alison's diary. Proof of Alison's blackmail against Byron is found in the diary. Aria rips out that page and keeps it. Mona regains trust from the school when she posts an apology video on the school's website with help from Jason DiLaurentis. Meredith, now the Liars new history teacher, is injured in a fire at the fundraiser. ' A' ending: 'A' is seen fiddling with someone's bike wheel, to which the owner falls off afterwards.
Credits
Adapted from Wikipedia contributors, Pretty Little Liars season 3. HTML formatting and reference markers removed.