Daniel Zuna, a Marine on leave, bleeds to death outside a club.

NCIS: LA's first stop is to investigate the Mixed Martial Arts gym where he spends most of his off-duty time. Kensi goes in posing as Zuna's new girlfriend.

She meets several members of the gym's team, most of whom were Marines who served with Zuna in Iraq.  After interviewing Zuna's former CO, it seems likely that Zuna's murder could be related to something that happened in Iraq.

Sam goes in undercover as an MMA fighter in order to get an inside scoop, but is targeted by another fighter who feels Sam is edging him off the team. 

We later learn the fighter is LAPD officer Marty Deeks, who has also been working undercover in they gym because it's linked to a major drug cartel.

Reluctantly, but later gratefully, Sam joins forces with Deeks to work both the drug case as well as the murder, in case the two happen to be related.

When another Marine from Zuna’s squad, is murdered, the two learn the truth of what happened in Iraq in order to solve the case.