Castle celebrates their 100th episode.

• Castle is stuck with a broken knee after skiing with Beckett.
• He believes he witnesses a murder in the apartment across the street and drives everyone crazy about it.
• Beckett & Co. solve a real murder with the death of an IRS agent. She was killed by her husband so he could get rid of her.
• Beckett decides to go over to the neighbors apartment to prove that nothing is going on over there. Castle watches from the window and sees Beckett getting attacked. He rushes over with Ryan and Espo but it turns out to be a surprise birthday party for him thrown by Beckett.