It will be from Kate's POV. It starts in ep 09 in s 04 (Kill shot). Kate is still with Josh but they have lots of fights. And he maltreats her. He rapes her but she doesn't tell anyone. She loves Castle but Rick doesn't know it.
Since a lot of you guys asked for it, here's a small sequel to my previous one shot "Twenty Questions" where you'll be able to read what happened after that night locked in the wine cellar.