The Jason Thing (situation)

"The Jason Thing" is a secret that Jenna and Garrett fear the Pretty Little Liars will find out. It is not revealed what it is until "Over My Dead Body," when it becomes clear that Jason and Garrett  slipped Jason a note the day after Alison  went missing that said, "I know what you did," leading him to question whether he murdered his own sister while high. For a full year after Alison's death, Jason questions whether he killed his own sister, keeping the note with him. He vigorously questions Spencer after he hears that she claimed that Ian confessed to killing Alison; such information would vindicate him if it is true, so he is very invested in verifying the claim. However, Spencer's vague answer leaves him wholly unsatisfied. In "The Devil You Know," he shows the anonymous note to Aria at Ian's funeral, after it seems that someone else killed his sister. But later, Jason figures out that he was not responsible and states this explicitly to Peter Hastings in the Rosewood Police Station. Garrett eavesdrops on their conversation and later reports to Jenna that Jason has figured out that he is not guilty.