A super hero is on the loose. Or is it really a villain? Castle and Beckett are called to the scene of a murder in an alley of a guy that attacked a young girl. Not only is it a murder but the victim, Tyler Faris who has is an ex con, was sliced in two with a sword thanks to someone who is dressed up as a comic book hero. Castle is all excited as he says "our killer is a super hero!" Just like a kid in a candy store, he tries to convince Beckett that there really are super heroes out in the world. Tony "the Butcher" Valtini is brought in for questioning and says that he was also attacked by that same person as he showed his L shaped scar on his butt cheek.
As Castle and Beckett head to a comic book store, his graphic novel 'Deadly Storm' is spotted on the rack. Beckett just rolls her eyes. The shop owner shows them that the killer's costume matches the one called 'Lone Vengeance" created by Sean Elt. They see that the knuckle plate matches the one that was seen at the crime scene so they head back there. As they see the knuckle plate on the ground, the unseen hero comes out of the shadows and grabs it. Esposito thinks he knows the killer's identity and gives Beckett the address. Chad Hockney is arrested but it turns out this is the wrong guy. Next, they look at Paul Whitaker, aka Sean Elt. Paul confesses to the killing and is ready to sign a statement. But Beckett realizes that he is covering up for the real killer after he messes up some information during the interview.
They soon find the address of the real Lone Vengeance and it turns out to be a female and someone in the police force, Ann Hastings. They discover that her father was killed during a holdup but she swears she didn't kill Tyler Ferris. She was trying to find out who did. Paul had confessed to protect Ann because he is in love with her. The evidence now points back to Tony the Butcher who wanted Tyler Faris dead. His fingerprints were found on the knuckle plate so they got the proof they needed. The real killer is finally found. It seems that there are way too many people running around the city with that same costume on.
As Paul and Ann walk away hand in hand, Castle and Beckett realizes that this couple resembles their own relationship. Could this be them in the future?
Back at home, Alexis and her dad have words over Alexis wanting to take the exact same classes as her boyfriend, Ashley while at Stanford. He is not happy with this because he thinks she is following Ashley's path, not her own path. In the end, Alexis takes her dad's advice. Castle is happy his daughter saw the light...until she mentions moving in with Ashley.
Stay tuned to see how this all plays out.