A woman is killed in the garage of a house belonging to Marine Gunnery Sergeant Holt Perkins.
Abby discovers that the mystery woman is Mallory Bleeker, a thief who's been camping out at the house while she was robbing it. She targeted Perkins' house because of his wife's status updates by which she learned that neither of them would be home.
During the autopsy, Ducky discovers a paper with a dragon's tooth symbol on it.
When McGee sends an image of the symbol to Delilah in Dubai, she informs him that the dragon's tooth is the emblem of a terrorist group. The group's MO is to monitor the activities of U.S. military personnel...and when they find out that they're deployed, they break into their homes. The group is run by a man named Omer Malik, who is a British national.
Tony and Bishop meet with Holt and Leandra Perkins to go over her social media friend list to see if they are aware of anyone they don't recognize. They find a name -- Henry Pecan. Abby discovers that Henry Pecan is an alias for Malik.
The team breaks into Malik's apartment where they find his innocent roommate who tells them that he's due to come back soon as he has to meet someone named Pistachio.
As they wait for Malik to arrive, Delilah and her team show up at the apartment, informing McGee that she is Pistachio -- which is a deep cover alias she's been using to try to get to Malik. She was posing as a disgruntled DOD employee.
McGee notices that Delilah's boss Special Agent Rafi keeps putting his hand on her. This causes him to worry about whether they are a little too friendly with each other.
During an interrogation, Gibbs learns from Malik's accomplice that Malik is obsessed over Delilah. He sends McGee to ask her to serve as bait for Malik.
McGee arrives at Delilah's safe house and confronts her about her being in the U.S. for a week without contacting him. During their argument, Malik shows up at her door. He points a gun at McGee and says "I heard shouting".
Malik ties up McGee and Delilah on the floor and tries to work out whether she is a double agent for NCIS or for him. McGee talks her into pretending to be on Malik's side.
Malik gets set to shoot McGee but Delilah tells him he doesn't have to. Malik agrees, pistol whips McGee, unties Delilah and orders her to shoot him as proof of her allegiance.
She points the gun at McGee, who encourages her to pull the trigger. She drops the gun, which McGee then informs her was unloaded. As they argue, Malik turns his back on Delilah. She pulls out a knife and stabs him in the leg, causing Malik to drop his gun.
As McGee and Malik struggle to get the weapon, Gibbs, Tony and Bishop break in and stop Malik.
Delilah informs the team that she's just been made Homeland Security team leader and that she'll be staying in the U.S.