The NCIS team investigates when a Marine dies while escorting a Vietnam veteran during an Honor Flight trip to Washington. They learn that he had been poisoned with botulinum toxin that had been injected by a tiny dart.
The veteran he had been escorting, Henry, proves to be a difficult man to wrangle. Gibbs and Vance set and unsuspecting Reeves onto keeping him company while the team investigates the case.
They connect the Marine to a crisis hotline, where a mysterious hacking had erased a bunch of calls. They link him to a first-time caller with whom he had spoken extensively.
They go to find her, only to discover she's dead, too, of the same toxin. Henry eventually identifies the killer from security camera footage; the killer works as the same flooring company as the second victim.
The second victim, Maya, was going to blow the whistle about some flooring contaminated by formaldehyde. The killer murdered her to keep her quiet, and killed the Marine to prevent him from telling anyone what she told him.
Gibbs helps Reeves understand Henry's issues, and Reeves secretly sets up a huge welcome-home ceremony for Henry when he goes home.
Meanwhile, McGee and Bishop try to discover if Quinn and Torres had a secret, torrid love affair at FLETC. Both categorically deny such a thing took place.