A young Marine reservist is brutally murdered in a fish aquarium, prompting OSP to investigate his death and the team are stunned when they discover that the victim's death is also connected to a case that Special Agent Mike Renko is investigating to a dangerous militia group, the Enhancing Law Enforcement or ELE. As they struggle to find the Corporal's murderer and recover six stolen anti-tank missiles while preventing ELE from launching their next attack against illegal immigrants, the whole situation soon escalates into a highly tense stand-off with a lot at stake and has Special Agent G. Callen caught in the middle. Meanwhile, Hetty is in Washington, D.C., attending a Senate hearing.
Enhancing Law Enforcement would later be mentioned in the Season 3 episode, Touch of Death.
During the episode, Hetty remarks to Director Leon Vance that the last she'd heard was that Macy was working out of a Quonset hut in Djibouti, although this was later contradicted in the NCIS Season 7 episode, "Patriot Down" where it was revealed that prior to her murder, Macy had been working with a task force in Marseille, France.
The episode name "Ambush" refers to the supposed ambush of the militia group during the episode.