Castle (Nathan Fillion) and Beckett (Stana Katic) investigate the possibly ritualistic murder of an accountant whose recent fascination with the occult and her death by stabbing of an ancient sword, not to mention the street footage of the woman being stalked by a monk (James MacDonald) and a college professor (Erik Jensen) who confirms the young woman was on a quest for lost Stonemason treasure, give Castle hope that they might have stumbled onto a real-life Da Vinci Code.
After catching up to the violent ex-con turned monk, who it turns out was trying to protect the woman from someone else following their victim, Castle and Beckett head to the church where the ancient clues lead the victim on the night of her death. The trail leads them to a blacksmith’s shop where Castle gets into a sword fight with an actor hired for an elaborate scavenger hunt for the New York Historical Institute which their victim was participating in on the night of her death.
Despondent over the entire mystery being nothing more than a game, the mystery writer returns home to get chewed out by Martha (Susan Sullivan) for his recent behavior while visiting Alexis (Molly C. Quinn) and Pi’s (Myko Olivier) new apartment for dinner. Wallowing even further, Castle makes a discovery that the real-life clues used to lend authenticity to the scavenger hunt which leads Castle and Beckett back to the church to unearth a hidden chamber where the young woman met her untimely end and the secret fortune that got her killed.
The real, then fake, then real mystery has some nice twists and turns especially as its nature feed’s Castle’s wild imaginings. Although Castle gets the elaborate mystery his heart was set on, finds the lost treasure, and helps Beckett catch their killer, the one thing the writer can’t do is patch up his relationship to his daughter who has had enough of her father’s disapproval of her (ridiculous) boyfriend.