Don’t diss your Dad in front of someone who lost theirs.
The fourth episode of Ao no Exorcist: Kyoto Fujouou-hen (Blue Exorcist: Kyoto Impure King Arc) picks up with Bon resolving to get answers from his father one way or another before spying on Juzo as he enters the lock-up to find someone has already gone on to try and steal the eye again. The way its framed makes Juzo seem like the traitor, but it doesn’t go any further as the opening begins. Then we get a brief shot of Shura’s rear as Rin finally manages to control his flames for once, but he’s only at the starting line since, if he loses control again, he’ll be executed. They even go out of their way to show us in a flashback the terms of his parole, but he’s optimistic.
Back with Juzo, it turns out that Mamushi was the traitor all along… to absolutely no one’s surprise. I mean, she was hardly doing much to hide it. But she gives the excuse that she’s doing all of this because Bon’s father and Mephisto are, in her eyes, the real traitors due to lying about the Left and Right Eyes of the Impure King, raising Rin, and using the Myoda’s sacred sword to them do it. So she’s throwing her lot in with Todo and plans to steal the eyes to seal them somewhere else.
The beautiful thing is, all of their superiors have cameras watching this. So they move to capture her as Juzo goes to stop her when, all of a sudden, everything starts shaking as Todo pops in. Everyone tries to reason with her, only for it to fall on deaf ears as she takes the eye with her and escapes with Todo.
Then we get to the meat of it, the confrontation between Bon and his father. Bon lays the blame on him for all of this happening, which he denies. But when asked to explain himself, he can’t and states it’s a secret he can’t tell anyone else. This sets Bon off and he disowns his father, which triggers Rin since that was the last thing he said to his father before the… unpleasantness happened.
Rin loses his cool at both of them before slugging Bon and tells him that they’ll regret letting things end this way. Then Bon makes it worse and Rin flares up as he tries to get him to apologize. Of course, then Juzo and Shura break up the conversation and she knocks him out as the episode ends.
From Mamushi’s words, it’s clear that Todo has been involved with planning this for roughly eight years or so. Bon’s father keeping quiet hints that whatever he’s hiding is pretty big, big enough to where he’s willing to sacrifice his bond with his child, and in the world of magic and demons that means a lot. Whatever this secret is will likely play a big role in the future, but for now we need to worry about Rin being thrown into isolation.