"One of the biggest mysteries of the second half of the season will be what happened to Mona? And the girls will have to piece together the clues and figure out what Mona had already figured out. Did Alison kill Mona? Did Alison kill Bethany? Those are also very important questions. There are all these mysteries that surround Alison, and the girls have to figure it out. And there will be clues left in Spencer’s honor."