Castle's latest creation isn't a new book. It's a s'morelet! A s'morelet, you see, is a graham cracker, marshmallows and a chocolate bar wrapped inside egg. The innovative entrée sends a grossed out Alexis scrambling for the door. But that's not the only reason she leaves in a hurry. Martha says Castle's little girl is in love. There's no time to digest this news. Beckett's calling. There's been a murder which, of course, means more s'morelet for Martha!
Daniel Goldstein is found shot to death in the park dressed only in his underwear. A print on the victim's wallet leads to a burly, scary career felon named Roland D'Andre. Call him D'Andre the Giant, as he did a wrestling slam on Esposito that puts the detective in a neck brace. But D'Andre's unregistered gun is a little too modern to have fired the 200-year-old bullet that killed Goldstein. Castle suggests the possibility of a time-traveling killer. After all, the victim did drive a DeLorean. Fire up the Flux Capacitor!
The victim's sister, Rachel, says her math genius brother was a bit of a dork who made a lot of money as a numbers cruncher on Wall Street. At the firm where Goldstein worked, Adam Murphy, says Daniel left work around 9:30. Goldstein's assistant, Julia Foster, says a lot of unhappy investors blamed her boss for a financial product fund that tanked causing hundreds of millions of dollars to be lost overnight.
Ryan and Esposito arrest a sword-wielding homeless man who is found wearing the victim's bloody Victorian-style shirt near Goldstein's Back to the Future-friendly car. Gunshot residue on the rest of Goldstein's clothes suggests he also had a weapon that was fired the night he was killed. Castle believes Goldstein had a secret life that led to an old-fashioned duel.
Castle and Beckett look into the disgruntled investor angle. Troy Kenworth's dad committed suicide after losing millions. But Troy says he didn't seek revenge for his father's death. In fact, he was grateful to Daniel Goldstein for getting him a job with a steampunk society. Castle explains that steampunk is a subculture that embraces the simplicity of the past while coupling it with the coolness of futuristic design. Got it?
The head of the steampunk society, Owen Peterson, shows Castle and Beckett a set of dueling pistols that had recently been fired, perhaps by fleeing steampunk society member Adam Murphy. But Adam doesn't get far when he runs smack dab into our favorite neck brace-wearing detective, Javier Esposito. Ouch!
Adam admits he unwittingly shot Goldstein to impress a girl. The two friends knew Daniel's assistant, Julia, would never get serious over a couple of geeks like them. They'd hoped the duel would show her the romance of it all. Daniel had calculated that no one would get hurt because antique guns are inaccurate in their trajectory. Castle and Beckett confirm this after firing off a few skewed shots with the antiques.
Beckett realizes the bullet that killed Adam Goldstein didn't come from the guns used by the duelers. There had to be a third shooter. Clothing fibers at the crime scene match the outfit worn by the steampunk society's newest staff member, Troy Kenworth. Guess he did want revenge after all. Case closed.
Castle is shocked to meet Ashley. Who is Ashley, you ask? Well, Ashley is the boy who is kissing his daughter on the couch. Thankfully, Castle only scares the kid and doesn't shoot him with the antique gun he holds. Castle is bummed that Alexis didn't talk about this with him. He used to be her go-to guy.
Of course, when Alexis does come to him for some thoughts on love, Castle can't get a word in edgewise. It's probably for the best. As Beckett told him earlier, "You do not mess with a teenage girl and her hormones." You don't mess with a homicide detective's hormones either. Guess that's why Castle doesn't press too hard to find out more about Josh, the handsome man who walks out of the station with Beckett. Hmm.