Now that Aria, Emily, Hanna and Spencer are in possession of the information that Alison left them, the girls decide that it's time to finally put an end to "A" and the mystery surrounding their friend. But without certain proof that their hunch is right, how can the girls catch "A" in his/her own game? And with little more than a video and hazy memories of the night that Alison disappeared, the Liars may have to turn to an unlikely source of information to help them put a stop to "A" once and for all – or so they think.