NCIS vyšetřuje vraždu podivného zbrojíře, seržanta Thomase Grimma. Muž byl nalezený s prostřelenou hrudí protitankovou střelou. Zprvu vše vypadá na to, že se tento muž stal obětí nějaké militantní skupiny, se kterou se nepohodl, když se jim pokoušel načerno prodávat zbraně z vojenských zásob. Byl skvělým boxerem, v námořnictvu byl od svých 17 let a vypracoval se na špičku.

Agenti NCIS však narazí na agentku jiné tajné služby, která byla do kšeftů se zbraněmi přímo zapletena a za smrt muže je přímo odpovědná. Grimm jí chtěl před smrtí předat zbraně, domluvil si s ní schůzku, na kterou však nepřišel.

A bunch of skateboarders find a body of a marine in the forest. When NCIS arrives at the scene, they discover that S.Sgt Grimm, an armorer who was on the boxing team, was killed with a missile. Kate meets Grimm's teammates in the gym, including Corporal McClain, who casually asks what happened and says she barely knew Grimm since they trained in the same gym.

Abby finds a serial number on the weapon that killed Grimm and traces it to the Armory where Grimm's had been stationed. Kate and Tony go to Grimm's apartment and discover his computer's entire hard drive had been reformatted. Abby is able to recover the erased data and finds the last file Grimm viewed – a picture of weapons on the floor of Grimm's apartment. Gibbs was also pulling out the pictures of the weapons on the Internet.

The team discovers Grimm had placed seven phone calls to a pawn shop the day he died. DiNozzo goes undercover at the pawn shop where the daughter of the imprisoned store owner is running the business now and is asks Tony to sell weapons to her. When they pull the guns on each other, she turns out to be an ATF Agent, Melinda Stone. Melinda informs Tony that ATF is running the operation. She talked to Grimm on the phone without knowing who he was and set up the meeting time but he never showed up.

Kate goes back to the gym and confronts McClain, who was transporting weapons with Grimm the day he died. McClain said she just signed out on the shipment that had the missing missile, Grimm dropped her off right after they left the base and she went to see her boyfriend, who was shipping off to Iraq. Later, Kate goes to Grimm's unit and talks to S.Sgt. Rafael who says everything is always according to the procedure including Grimm signing in the weapons three days earlier.

Gibbs tells Melinda to call the buyer and set up the meeting where he would be going undercover.

Kate is now interrogating Rafael, who failed to mention that he attended the Armory School with Grimm and they kept in touch. After he is shown a picture of Grimm's body, Rafael admits that Grimm talked him into signing out the weapons as a one-time deal that would be good for the three people involved. Corporal McClain's alibi doesn't check out after her picture is shown to the hotel clerk and her boyfriend says that they broke up a month ago.

Melinda and Gibbs meet with the buyers – the local militia, who take them along to test the weapons and dispose the tracking device that was in Gibbs' watch on the way. When militia starts checking out the weapons, the first one is fine, but the other models are not in working condition. When Gibbs offers to lower the price he gets shot by the militia's leader. The militia leader makes Melinda call her supplier for more weapons. The supplier turns out to be McClain and when she goes to the storage area she is arrested by Kate and Tony.

When the militia leaders receives a call, he helps Gibbs up and it is revealed that it was an undercover operation with ATF really watching Melinda Stone since many things looked wrong during her last operation. It's also revealed that Grimm was killed during an argument.