A mob doctor is killed by a bad Santa and Castle and Beckett are working the case. Since it's a mob case, no one wants to talk to outsiders. Beckett and Co. pinned the wrong guy. Luckily, for us Castle knows the Don of the family and when the Don asks you to do something, you do it. The Don asks Castle to find out who killed the mob doc because he was literally family to him. They swore a blood oath and Castle gets down to work. He finds out that the Don's daughter and that she knows something. The Don didn't like hearing that and would have killed Castle if it weren't for the daughter stepping in to clear everything up. It was a Romeo and Juliet scenario where the two families were against each other, but the kids loved each other. It was thought that the Romeo was dead, but actually he was just in hiding because bad Santa was trying to kill him and simultaneously was taking over his families turf and making people pay bad Santa . Being a cop is fun and all, at least in this show, but when you don't have to follow the rules well sometimes that can leave to better and faster results. A tech lady mobster tells the team where the payments to bad Santa were going to and with video cameras, they found out who bad Santa was--a cop. Not just any cop, but the cop that was helping the team out on this case. I guess not all cops follow the rules. The Don now owes Castle and I'm pretty sure Castle will cash in his chips with his story-arc.
On the relationship side, Lanie and Esposito had to act like they were engaged because Lanie told a white lie that spiraled out of control. But everything turned out find for them in the end, but because Lanie and Espoito were acting like an engaged couple, it made them realize that they don't have the great love that Castle and Beckett have. So they broke up, again. How many times is this? I'm pretty sure this one is for good. It definitely shocked me, but I wasn't hurt by the revelation so I'll survive.
The sad thing about working with the mob is that you become the enemies of the cops. The dirty cop got killed and someone complained to the mayor and the mayor had to do something. Castle lost his job, y'all! Beckett still doesn't know and I wonder if we'll see him tell her. I kind of want to see it, but am afraid they'll skip over it. We did, however, get to hear Beckett's lovely poem she wrote about her year. A sweet send off, I guess.