The 100th episode of Castle has a very Rear Window-ish feel to it. Our favorite mystery man is stuck in wheelchair after breaking his kneecap while skiing. He uses a pair of binoculars Alexis got him as a joke to check out the residents in the apartment building across the way. He sees two writers hard at work. The male scribe looks suspiciously like series creator Andrew W. Marlowe. The other writer is his wife, Terri Edda Miller. The two of them co-wrote this particular episode.
As he continues to do the Peeping Rick thing, Castle catches sight of a young woman engaged in some afternoon delight with a young man with a black hat. The flirty lady shuffles her lover out the door when her main squeeze comes home unexpectedly. Too bad her fleeing paramour left his stylish chapeau behind. The apartment tenant knows this lonely lady is being unfaithful. He takes a knife and follows her into the bedroom. The blinds are closed as an attack take place. Holy MacGuffin! Castle believes he just witnessed a murder.
Speaking of murder, apartment building security guard Selena Rigas notices a figure walking down an outside alley on her surveillance monitor. No face can be seen thanks to the bright light shining above the neck. The figure attacks a woman with a bat. A second security guard rushes into the alley, but he's too late. Clara DeWinter is dead in a dumpster. The killer's car is seen on the video footage. Clara's husband, Gavin, says his IRS agent wife had been threatened by a tax cheat she took down. Beckett's investigation gets put on hold when Castle calls about the murder he saw through his binoculars.
Beckett, Esposito and Ryan check out the apartment across from Castle's loft. There's no sign of foul play. The man in the apartment says he had a fight with his girlfriend. They broke up before she left the apartment. A call to the woman confirms that she's alive and well. Castle is thoroughly confused. He awakens in the middle of the night to spy on the apartment. Beckett believes he's obsessing over an imaginary murder because he's bored. Castle begs to differ. He later sees the guy washing away blood in the sink. He also notices that some rope and a tarp have been purchased. Castle is truly dismayed that Beckett still doesn't believe him.
Castle continues to keep watch on the apartment across the street. He catches sight of the tenant dragging a rug across the room. There could be a body inside. Alexis joins her dad in his snooping venture since his murder suspect has washboard abs. She's a little freaked when she sees the guy donning rubber gloves and a respirator mask. Castle enlists Alexis to be his lookout as he breaks into his suspect's apartment. He finds what look to be blood spatter on the closet wall along with cleaning supplies. Castle hides under the bed when the apartment tenant returns. He manages to sneak out with a bag of remnants from the shredder.
Regarding Beckett's murder case, Dan Renner is the man who threatened Clara DeWinter. He has a vehicle that matches the make and model of the one fleeing the murder scene. Renner says his car was seized weeks ago. He alibis out for the murder, but says Clara was looking to him for help. She wanted to disappear. The victim had ties to a crime boss named Tommy "The Shark" Valentino. The mob man says Clara was looking to get away from her husband who recently took out a large life insurance policy on his wife. Gavin DeWinter alibis out, but that doesn't mean he didn't hire someone to kill his wife.
Beckett must, once again, press pause on her investigation to listen to Castle's claims about the murder only he saw happen. A storage unit receipt suggests the body is being hidden there. Beckett happens to be in Castle's loft when the missing woman's lover returns. She has a chat with the guy who admits that he hasn't heard from his lover in days. Castle convinces Beckett to join him as he breaks into the storage unit. The place is clean and they are caught red-handed by a security guard. Captain Gates reads them both the riot act. She tells Castle to stay away from his neighbor or else the guy will press charges.
Beckett puts on a beautiful dress to celebrate her boyfriend's birthday. Too bad Castle is still obsessed with the apartment tenant. He thinks the dead woman's body is stuffed inside the refrigerator. Beckett has had enough. She finagles her way into the apartment to prove there's been no murder. When she makes a move to the fridge, the man pulls her away. He grabs a knife as the two of them struggle. Castle watches in horror as the lights go out. Police storm the scene as Castle frantically hobbles up to the apartment. Ryan and Esposito break down the door. A worried Castle follows them into the apartment completely unprepared for what's inside.
Surprise! Castle is shocked to see all his friends standing before him. There's a "happy birthday" banner on the wall. The apartment tenant, the missing girl and her hat-wearing lover are also there. They are all actors. The entire murder scene was a setup. Beckett wanted to do something special for her partner for his birthday. Castle struggles to process what's happening. Everyone worries that they may have gone too far until the birthday boy says this is the greatest birthday gift of his life! The icing on the cake is that Castle even helps Beckett solve her real murder case.
Beckett realizes her case also involves a staged murder. The video surveillance footage had been tampered with by someone who had access to the feed. The security guard, Selena Rigas, played the part of the murder victim in prerecorded footage that was dropped into the live footage. She was having an affair with the victim's husband as the two of them looked to cash in on the life insurance policy. Case closed. When Beckett returns to Castle's place, they estimate that they've solved about 100 murder cases together. That doesn't include the earlier fake one. Castle vows that he'll get Beckett for what she did to him as he wheels her off into the bedroom. Happy 100!