Pretty Little Liars Season 5 Episode 12
Taking This One To The Grave
The liars enlist Mona's help to find out what Ali is telling the police.
Although initially reluctant, Mona and Lucas hack into the police computers to find out what happened when Alison was given a lie detector test. Mona feels certain than Alison is a sociopath who has been playing with her friends this whole time as A.
The police interview shows that Alison doesn't reveal much incriminating information about herself or the girls, but the police's questions suggest that they are more interested in Spencer than anyone else.
Knowing the police will probably be arresting her soon for Bethany's murder Spencer breaks into Radley with Mona and takes a bunch of files and tape recorded sessions about Bethany.
They discover Mrs. DiLaurentis was having an affair with Bethany's father. Mona goes through the files and listens to the tape at home. She calls Aria to say that she thinks Alison knew about the affair and lured Bethany to Rosewood. The girls arrive at Mona's to find blood all over her home.
The police determine that although there's no body, Mona was murdered.