Martha warns Castle to be on his best behavior as he pays a visit to his daughter in the apartment she found with Pi. It's a quaint little place with a door serving as a dining room table and chairs Alexis found in a dumpster. It's a suitable setup for many fruitarian meals. Castle offers to give the kids a couch he has in storage. Pi wonders what color it is. Castle's answer: Free. The evening is a total disaster. Castle needs a good murder to take his mind off things. As luck would have it, a 28-year-old accountant named Susanna Richland looks to be the victim of a ritualistic killing.
The murder victim has what looks like bloody stigmata marks on her hands. She was stabbed through the throat with a dagger-like instrument. It was actually a sword. The woman's apartment is filled with photos of religious symbols and pagan elements. Castle believes Susanna may have been interested in the occult. Her only known family member, her cousin Henry Collins, says the victim called him saying she had something to talk about. Traffic cam footage shows her being followed by a man who looks like a monk. Castle believes this case is turning into a real-life "Da Vinci Code."
A professor the victim contacted says some of the symbols she was researching served as markers for buried treasure like some missing half-dimes, the first coins minted by the U.S. Treasury. The professor shows Castle a letter Susanna had from Theodore Rose, an 18th Century mason who mentions a great secret enshrined within the city. It contains clues that could lead to a treasure. As for the mystery monk, his name is Benjamin Wade. He recently had brain surgery. He's also an ex-con. Beckett believes this makes him a "felonious monk."
Esposito and Ryan bring in Wade, who claims he was trying to protect Susanna Richland. He met her in a chapel and claims another aggressive individual was asking lots of questions about her. The guy got into a cab with a banner ad for Wicked. Beckett orders an incredulous Ryan to run down this lead. She then takes Castle to the chapel where Wade met the victim. A symbol matches the one carved into her hand. This lead takes them to an old closed blacksmith shop where a masked man with a sword is waiting.
With Beckett checking out the back, Castle must grab a sword to engage in a duel with this stranger. There's a mighty clash of steel on steel. Eventually, Castle gets the upper hand. His opponent reveals a symbol upon defeat. The guy claims he's just an actor taking part in a scavenger hunt. He recognizes the murder victim as one of the contestants. Nolan Burns, Director of the Cultural Institute, says he was just trying to raise awareness of the symbols through his fundraiser. He based the game on Theodore Rose's legacy. Castle is disappointed. He truly thought this case would lead to a hidden freemason treasure.
A man who was going by the name Tom Stevens was part of the game and may be the real killer. Ryan's detective work on the cab lets the team know that Susanna went back to the chapel. Castle believes she figured out that the clues in the game actually led somewhere. The killer scraped the symbols off her hands because he didn't want anyone else to figure it out. Castle finds access to a secret room inside the chapel. It's the crime scene. Hidden in a crypt is a container filled with the valuable missing half-dimes. When Castle removes it, a safety mechanism traps him and Beckett inside the hidden room. Oops.
Beckett is able to get enough bars on her cell phone to call for help. Nolan Burns becomes the prime suspect in the case, as his game was a cover for a real search for the missing coins. The guy is no killer though. He lets Beckett know that the victim was a direct descendant of Theodore Rose. The legend of the missing dimes was part of her family lore. This leads Castle and Beckett to the real killer, the victim's cousin, Henry Collins. The guy signed up for the game to track Susanna's progress. They argued about what to do with the coins when things got out of hand. Case closed.
Castle tries to make amends with Alexis, but she's not ready to forgive him. She won't even let him inside her apartment. Alexis says she doesn't know if Beckett is the right person for him, but she accepts her because she makes him happy. All she wants is for her dad to accept Pi. Castle promises he'll work on it. He offers to take his daughter out for some makeup ice cream. Alexis turns him down. She needs time to be mad. She also feels Castle needs time to accept her relationship with Pi. She closes the door leaving Castle standing in the hallway unsure of what to do next.