Usually, Richard Castle's overactive imagination is a good thing — so good that as he helps his NYPD buddies solve scores of murders, you often wonder how they'd ever get along without him. In the set-up for Castle's clever lark of a 100th episode on ABC (Monday, 10:01/9:01c), they're forced to go solo as Castle (Nathan Fillion in rare form) stews in boredom in his apartment, nursing a busted leg. Until he picks up his new birthday binoculars and goes all Jimmy Stewart-in-Rear Window, convinced he's witnessed a murder across the street, eventually drawing his beloved Beckett and intrepid daughter Alexis into the Hitchcockian-homage intrigue. The more agitated Castle gets, the more skeptical everyone else becomes, and as the twists and comically suspenseful close calls pile up, leading to yet another chewing-out by that spoilsport Capt. Gates, we're treated to an entertaining object lesson in the "seeing isn't believing" playbook. Well done, including the timely subplot involving the murder of an IRS agent which, even when chair-bound, Castle can't help inadvertently helping his friends figure out.