Tonight's episode begins with 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.

Moreover, they think he could be supplying them to local gangs. 

An investigation into Dobbs' life reveals that 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 there, Callen and Kensi investigate how the guns were getting in to the U.S. 

The guns were knockoffs 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.

Gang members stationed with the military in Afghanistan were hiding heroin in the side panels of Humvees being shipped back to the US.