Castle is home, mostly alone, in a wheelchair after a skiing accident, bored out of his wits just before his birthday. After a while, he actually picks up a pair of binoculars Alexis gave him to play Rear Window. Indeed, right across the street a foxy resident betrays her studly husband with a younger hottie. Shortly after, the cheated husband finds his rival's hat, wields a knife and seems to dispose of the corpse while she isn't seen again. Yet his NYPD team mates only reluctantly investigate and can't find any proof. Castle becomes obsessed with the possibility that he witnessed a murder, while Beckett grows increasingly upset with his mania and sets out to find out the truth one way or the other. That's when things take a surprising and shocking turn...