In Castle’s 100th episode, everyone’s favorite writer is down for the count after a bad skiing accident, and after two weeks stuck at home in a cast, he’s got a little cabin fever.

“He’s not bed ridden but certainly apartment-ridden,” says Nathan Fillion, who calls the hour one of the show’s classic, lighter-toned episodes. “He’s two weeks into being incapacitated, and it’s all he can do just to stay sane. [Then] he witnesses a murder across the street, and he’s having a hell of a time convincing everyone he’s not just stir crazy and seeing what he wants to see.”

So what’s the truth? Our money is on murder — since these things have a way of gravitating toward Castle and Co. But you’ll have to tune in April 1 to find out for sure.

Meanwhile, Fillion says there’s another reason fans should consider this milestone episode a can’t-miss hour: “Let’s say that we get an opportunity to see just how well Beckett knows Castle,” he teases.