On Teen Wolf Season 6 Episode 2, the Ghost Riders strike at the school. Corey and Mason see them, while Corey is making them appear invisible. Right after the Ghost Riders leave, Corey and Mason forget everything, but they suspect that they are forgetting something.
The pack begins to realize that something is off. Scott, Malia, and Lydia each have something that triggers this feeling that something is missing. Deaton suggests that Scott is suffering from something like Phantom Limb Syndrome, and he encourages Scott to go to sleep and let his subconscious work at remembering.
Scott ends up in the woods, specifically where he was first bitten. Malia realizes she is reverting back to being a coyote and that someone used to chain her up on the full moon. Lydia is still struggling to remember who she was supposed to meet in the morning at school.
Lydia stares at a piece of glass, which somehow it is supposed to help her remember, and it does. She writes the word mischief over and over, in the shape of a name: Stiles.
Liam and Corey work together to try to find a trace of the Ghost Riders. While invisible, they spot a blank library card. Corey is able to bring that card back from the invisible plane of existence, and when Liam runs it through the system, they remember. Jake Sullivan. All of their memories of him come back, and the library card is no longer blank.