A drone strike on a bomb-making compound in Afghanistan reveals the body of a recently retired Marine, leaving the NCIS: LA team to investigate why the Marine returned to Afghanistan and whether or not he betrayed the country he fought for.

When cyanide in a communal water jug lethally poisons a lieutenant and leaves a Navy pilot in serious condition, the NCIS: LA team investigates whether this is an isolated incident or an early test for large-scale water contamination.