At the DoD high school aboard the Marine Corps Base, Quantico, Virginia, students and teachers fall asleep while in class; Gibbs and company, augmented with three short-term interns from a nearby university, investigate; Tony and McGee find a cylinder of gas connected to the ventilation system. After the evacuation of the building, one student, Rebecca, is absent; the team start looking for her, and they call the FBI. Gibbs meets with Mason, Rebecca's paternal grandfather, who starts acting on his own, and who presents a curious background; Mason slips away, but the gang later find him; then Mason tells Gibbs about his true history. Abby and her intern produce. Attention turns to the teacher, who turns out to be something else. Gibbs, Tony, McGee, and Ziva meet with Mason and the teacher, and the gang find Rebecca.