Hell hath no fury like a sexy police detective scorned! In the season premiere, Castle becomes the prime suspect for Detective Beckett's murder investigation and the target of her anger after rejecting her at the beginning of summer.
After turning Beckett down during last season, Castle returns in the most dramatic way he can: showing up as incriminating as possible at a murder scene. Unfortunately for him, Beckett isn't very understanding of his situation especially since she's still feeling rather bitter about the rejection. And of course, he insists on his innocence and has no clue what he's done to piss her off.
Luckily, Castle's innocence is indeed proven and he gets back on the case though a bet with Beckett. Two more murders later and speculation about a meth ring and counterfeiting begins. Castle even manages to give a shout out to Breaking Bad!