The team investigates the murder of Lance Corporal Ramon Vega when his car explodes while being chased by police. He placed a call just as he died.
To his brother, Eduardo.
In his car they find traces of cobalt-60, a component used in constructing a dirty bomb, or improvised explosive device with radioactive material.
With the U.S. Senate about to vote on a defense bill the timing of a dirty bomb out there about to explode sends the team scrambling to track it down.
Roman was being investigated by a private intelligence firm that was contracted by the Pentagon, as government staff is stretched thin.
That firm tied him to Libertad Nuevo, a terrorist ring in Peru.
However, when he shows up alive they realize the victim in the car was not Vega but someone else, a man actually hired by the intelligence firm to scare the city into thinking a dirty bomb was in play.
Thus influencing the Senate to pass the bill to ensure them contracts.
The intelligence firm made up the existence of Libertad Nuevo and used Roman as a pawn to take the fall as a homegrown terrorist.
Intriguingly, M. Allison Hart, the no-nonsense attorney with a thing for Gibbs, returned to represent Roman Vega and his brother.
Gibbs, of course, knows that she also represents Colonel Bell, who Gibbs sent to Mexico to face bounty hunting charges late last year.
Things heat up between Hart and Gibbs as she appears in his house at the end, and he again confronts her about taking cases that cross his path.
They almost kiss as the credits roll.