Alexis has moved in with Pi and Castle and his mom pay them a visit at their apartment. Alexis has invited her father and grandma over for dinner to see their place. Castle is in a grumpy mood and his mother asks him not to be judgemental. Castle is at his sarcastic best and he later tells Beckett that the dinner was a disaster. But, according to Castle, the real disaster is Alexis moving in with Pi.

Castle needs a murder so that he stops obsessing about Pi and Alexis' decision to move in with him. And, there is a murder of a young account, Susannah, waiting for him. The body of the dead girl suggests of crucifixion. Castle notes that it may be some kind of ritualistic sacrifice, Beckett says may be the killer wanted to make it look like one. Lanie points out to the lack of blood and says that she must have been murdered somewhere else and her body moved to the spot where they found it. "May I suggest sacrificial altar," Castle says, but Beckett stops him from further talking about that.

Beckett and Castle go to the murdered victim's apartment and the landlady tells them that something was up as she started keeping odd hours and being out till late hours, which was quite unlike her. From the victim's boss, Esposito (Jon Huertas) finds out that she was acting odd, lately and had taken a week off to settle personal matters.

At the apartment of the victim, Beckett sees pictures of religious icons and pagan symbols pinned on a board. There are also lots of books on medieval history, too. Castle remarks that the girl was into a cult, which her cousin, Henry, denies. The cousin tells Esposito that they used to meet once in a while as he was her only family. However, she had called him a day before the murder to say that she had something that she wanted to talk to him about. But, she got interrupted and never called up, again to tell the details.

Ryan tells Castle and Beckett that they found a MetroCard in Susannah's pocket and the card was swiped the night before on a bus. Ryan also finds a video footage of Susannah walking down a street and a monk following her. The time stamp on the video suggests that the monk was following Susannah right at the time of her murder. Hearing all the details, Castle believes that it is "Da Vinci Code" style murder conspiracy. The video footage does not reveal the face of the monk. Beckett spots a surgical scar on the monk's bald head.

Esposito informs Beckett and Castle that the victim had made call to a University's professor, a day before her murder. The professor is an expert on ancient languages and symbology.

Lanie informs that Esposito that the victim was stabbed with a sword, and ink residue has been found in the wound of the victim's hand. Lanie tells him that it looks like she drew a symbol on her hand. Lanie shows Esposito the image of that symbol, which he forwards to Beckett.

Beckett shows the symbol to the professor who does not remember Susannah talking about it. When informed that the girl was murdered, the professor shows them a picture of a letter that Susannah had with her. She had wanted to know if the letter, which was written in 1798 by Theodore Rose, was connected to symbols. The letter mentions about a "great secret" enshrined in the city. According to Castle, the letter is possibly a key to that.

Ryan calls up Beckett to inform that the scar on the monk's head is possibly from a stereotactic brain surgery. The Monk, Benjamin Wade, has a record of attempted murder. Ryan and Esposito arrest the man for the murder of Susannah. Beckett interrogates the monk on why he was stalking the victim. Also, she tells him that his monastery has an office at the same place where they found Susannah's body.

The monk says that he was not stalking her but was trying to protect her, and that he has turned over a new leaf. He had found her at the monastery chapel and that she had talked about the place being special to her family. The monk also mentions about a man who had showed up and that the man was upset and aggressive, and was asking questions about what the victim was doing there. He had thought that the man would follow so the monk says that he followed her and saw her get into a cab.

Beckett and Castle check out the Chapel and see the symbol that is carved in the base-stone. The next stop is a blacksmith shop. There are swords of all kinds on display inside the shop. Beckett looks around while Castle ends-up having a sword fight with a man wearing a mask. The man before pulling out his sword said "you've trespassed into a secret, beyond your comprehension and you'll pay with your life.

Castle believes that he is good at a sword game and proves that. But, the man he defeated turns out to be an actor who was following a script. The script is for a scavenger hunt game. Susannah was one of the contestants and had visited the place. The cab driver reveals that Susannah never left the neighbourhood of the monastery on the night of her murder. She had asked the cab driver to drive around the neighbourhood for some time. He had dropped her back at the chapel.

Castle's mom asks him to change his point of view regarding Alexis' decision to live with Pi. The 'point of view' makes him re-look at the symbols again. He creates a complete image by putting the symbols around the symbol that was found on the hand of Susannah. The complete image leads Castle and Beckett back to the chapel as the image resembles that of a carving on a wall, there. Castle is able to open that wall, following the clues written in the letter by Rose. Inside the coffin kept there, Castle and Beckett discover a pot of coins, with the time-stamp 1792. According to Castle, they are the first coins minted by the U.S. treasury. The half-dimes are worth the fortune. It turns out Susannah is a direct descendant of Rose and had heard about the treasure.

The finger of suspicion first point at the organiser of the scavenger hunt, Mr Burns. But, Henry, the cousin of Suannah, is the one who killed her. She wanted to give-up the coins to the nation after finding the treasure, while Henry wanted to keep them as he wasn't doing great money-wise.

Castle is unable to make peace with Alexis. She feels that Castle has not accepted her decision like she has accepted his decision to marry Beckett though she also had apprehensions. Alexis also complains that she came to know about his proposal through grand-mom.