When Marine Gunnery Sergeant Wendall Dobbs is gunned down in the streets by a gang wielding AK-47's, NCIS: LA suspects Dobbs could be involved with illegally importing automatic weapons into the country and supplying them to local gangs. Investigation into Dobbs life reveals that he is above suspicious and traded in his combat post for an administrative position in order to mentor his younger brother, Darryl, who has been hanging out with a gang. Sam goes undercover to pose as a cousin and Darryl introduces him around. As Sam gets involved in gang business, Callen and Kensi investigate how the guns were getting in to the country. Turns out, the guns were cheap knockoffs that came from an outfit in Florida, and though they were used to kill Dobbs, they weren't the reason for his death. In fact, Dobbs had stumbled on a heroin smuggling scheme whereby gang members stationed with the military in Afghanistan were hiding heroin in the side panels of military Humvees being shipped back to the US.