(Warning: this post contains spoilers from Episode 5.) While the biggest question on 13 Reasons Why is why did Hannah Baker decide to end her life, there are a few other mysteries that come up in the show as well. One in particular that comes to mind is: what happened at Jessica's party on 13 Reasons Why? Prior to Hannah's death, she put together cassette tapes with each side explaining the 13 reasons (and the 13 people) that contributed to her decision to commit suicide. At the start of the show, Clay Jensen received the tapes, but he's not the first one who's had them. A group of his classmates who have already listened to them are worried about what he'll find out when he gets to the tape about Jessica's party and what he'll do with the information once he finds out.
Those classmates include Hannah's first initial love interest, Justin, Hannah's former best friend Jessica, Justin's friend Zach as well as Marcus, and Courtney — all of whom have a tape dedicated to them. While they each have something to hide since they're included on the tapes, there's something specific that happens at Jessica's party that is apparently worse than anything else Hannah has to say. Or at least that's what her classmates seem to think.
So what happened? Well obviously Jessica's involved somehow since it was her party. Additionally, it appears Justin's also connected, as is his friend Bryce whose house Justin was crashing at for a while. The difference is Bryce doesn't seem to have any clue that something's even going on and apparently his friends don't want him to.
Jessica is adamant about the fact that what Hannah says on the tapes are lies, but it's also possible that she's in denial about whatever it is Hannah said. This seems especially likely because Justin and his friends seem to know more than they're saying and it looks like they don't want Jessica to believe what's on the tapes in order to protect themselves.
If you read the bestselling novel of the same name by Jay Asher, then you know what they're trying to hide. However, if you haven't read the book don't continue reading. Book spoilers abound!
In the book, Hannah explains that during the party Jessica became unconscious, presumably from drinking too much, and Justin allowed Bryce to have sex with her. This is explains why Justin wants Jessica to believe it's a lie, but not so much why they want to hide from Bryce. If Bryce did it, he knows he did and he's probably on the tapes himself, so he's going to find out eventually anyway, right? So why hide it?
The truth is bound to come to light eventually once Clay gets to that particular tape, and when he does, I have a feeling it's going to come with some serious repercussions.