A stone gargoyle falls from the roof of an apartment building crushing Will Medina, the associate curator at the New York History Museum. Medina's boss, Stanford Raynes, says Will was the leader of an expedition that discovered the crypt of Kan-Xul, the Mayan king. Castle opens the sarcophagus of Kan-Xul, only to be scolded by Rachel Walters, the expedition's mummification expert. When told of Medina's death, she and Rupert Bentley, the financier, fear that what really killed Medina was not a gargoyle. It was the curse of the mummy.
Rupert reveals the fate of the expedition members who gazed upon Kan-Xul: a grad student, Nicole Graham, was a mauled to death by a jaguar and another member died of dengue fever. It seems whoever looks upon the mummy ends up dead. That's a bummer for Castle, who recently went eye to eye with the ancient king.
Seeing how antsy her partner is about the curse, Beckett decides to have a little fun. She loosens the screws from a chair so that it collapses as Castle sits. With a little help from the bomb squad, she has the coffee machine harmlessly explode as he goes to fill his mug. Castle is freaked. He's especially petrified when an attack dog takes a chomp out the backside of his pants during an arrest. Even though Beckett had nothing to do with that one, did anyone notice the furtive glance she gave to Castle's exposed backside as he walked away?
It turns out that Medina was dating Rachel Waters. She kept the relationship a secret because she was worried about getting fired. Stanford Raynes blamed Medina for the death of Nicole Graham. Raynes claims that Nicole died in the jungle because she had gone to hook up with Medina. He was angry but was in the museum at the time of the murder.
Rupert Bentley seems to be enjoying the spooky press that's the museum is receiving due to the mummy's curse. Castle wonders if he murdered Medina to boost ticket sales. Beckett dismisses this theory as a little too Scooby-Doo and she's not Velma. Castle says, "Velma? Are you kidding? You're Daphne. You're hot, smart, not aggressively brainy, but long legs, short skirt..." Well, you get the idea.
A retired drug trafficker named Norton Grimes was dealing with Medina to procure one of the slave girl mummies for a buyer overseas. Suspecting that Grimes is lying, our team heads back to the museum to do a swab test. If the slave girl mummy tests positive for drugs, Grimes is likely the killer. However, when they open up the sarcophagus, they see that the mummy is gone.
During an appraisal of the mummy for a foreign buyer, Carbon-14 testing authenticated the sarcophagus and wrappings, but the mummy itself was too young to date. This "slave girl" had only been dead four months. When Castle hears this, he knows the identity. It's Nicole Graham, the first victim of the "curse."
Nicole's body was never found; the authorities presumed she was killed by an animal based on scant evidence. For the killer, mummification was the perfect way to hide the body: just ship it out with the other artifacts. Medina found out about it and the killer murdered him, blaming both deaths on the curse. Beckett explains that the expedition just happened to include the foremost expert on Mayan mummification: Rachel Walters.
Rachel denies killing Nicole, as she had no motive. They both had a crush on Medina, but they were also friends. Once Nicole realized that Medina liked Rachel, she backed off. But Stanford Raynes had a creepy crush on Nicole, and he's the one who taught Rachel everything she knows about mummification. She says that the mummification process leaves DNA behind; she'll gladly have hers tested, but doubts Raynes will do the same.
At the museum, Castle and Beckett confront Raynes. Beckett spins the story of a spurned love for Nicole and a rage that led to murder, cover-up and more murder. Castle says, "This is the part where you say...and I would have gotten away with it, too, if it weren't for you meddling kids." Raynes makes a run for it, but falls down a flight of stairs right in front of a portrait of Kan-Xul. Coincidence? Or the curse of the mummy? Beckett's response: "Ruh-roh!"