A dead Marine is found in a remote clearing, seemingly impaled on a tree stump when Gibbs (MARK HARMON), Kate (SASHA ALEXANDER), Tony (MICHAEL WEATHERLY) and Ducky (DAVID McCALLUM) are called to the scene to investigate. Ducky determines that the man, whose bruises indicate he was also a boxer, was killed within the last 24 hours. On site, Tony also finds a shell casing that is a remnant from a high explosive, multi-purpose assault weapon launcher – typically a terrorist weapon. The weapon’s explosion is probably what killed the marine.
The agents identify the Marine as Staff Sergeant Thomas Grimm, who was an armorer attached to the maintenance center at Quantico and responsible for weapons inventory and custody cards. Then, Kate interviews Grimm’s fellow male and female boxers, including Corporal Patty McClain. While McClain initially says she barely knew Grimm, the agents realize that she and Grimm were making deliveries of weapons to Camp Geiger, where Staff Sergeant Phillip Rafael checked them in.
Meanwhile, Abby (PAULEY PERRETTE) puts the shell casing under close scrutiny and realizes it matches the one that killed Grimm and, from his personal computer hard drive, uncovers that he had a host of weaponry being primed for the black market. Kate also brings in Rafael, when she discovers – despite Rafael’s stoic stance – that one of the weapons was missing, which also ended up being the one that killed Grimm. Confronted with that information, McClain insists she has an alibi which seems to be sturdy... and Rafael won’t budge on divulging any information that might incriminate her, Grimm or himself.
When Gibbs discovers that Grimm’s recent phone calls include a local pawnshop that might have been buying the weapons, he and Tony set up a sting of sorts. Tony goes undercover to try and sell illegal weapons to the shop’s owner, Malina Simms. But just as their trap seems a surefire bet, Malina draws a gun and reveals shocking information: she is actually an ATF agent. She had previously set up Grimm’s weapons with a seller, and didn’t know he had been killed. Now, Gibbs believes that perhaps the seller is the culprit.
With Malina now also embroiled in the case, Gibbs demands that she join their investigation to uncover the killer’s identity. He convinces her to set up a new meeting with her “buyers,” with Gibbs posing as the new seller. In that arena, Gibbs thinks he might be able to ascertain who killed Grimm. With a batch of weapons for the sale, Gibbs and Malina rendezvous with the buyers – including the unstable Roger Cook and fellow overzealous militia members – who then kidnap the two to go and test the weapons.
While Gibbs and Malina are in that ordeal, Kate confronts Rafael again and presents the scenario that he was receiving a payment for overlooking weapons that didn’t get checked in at Camp Gieger that Grimm and McClain would eventually sell. Finally, with the threat of his conviction looming, Rafael admits to the scheme he, McClain and Grimm had worked out. She then makes a startling discovery: McClain’s alibi doesn’t, in fact, check out. With that information, Kate heads off to arrest McClain.
As Gibbs and Malina are locked into a tense situation with Cooke, who is testing the weapons for sale, he then suspects that he has been set up and – to Malina’s horror – he shoots Gibbs, who is bleeding profusely but alive.
As Malina freaks out over the turn of events, she agrees to summon another seller that could correct the situation. She makes a call to McClain who, after collecting the weapons for delivery, is apprehended by Kate. Kate then calls Cooke on-the-scene... who reveals to Malina that he is actually an ATF agent working undercover – and with NCIS (and an instantly recovered, back-on-his-feet Gibbs) – to nab a shocked Malina. With Kate’s findings, they have determined that Malina worked with McClain to take in money from weapon sales and cut Grimm out of his share of the money by killing him.