Information about the location of nuclear detection devices in Los Angeles county is stolen. The men who stole the information were both killed and then taken from them. Hetty worries that someone from her past that she let go free is involved. That could end her career.
A sensor is activated and it was a diversion and test of their response. They killed the man who was the nuclear trigger to activate the sensor. NCIS realizes it is a test and follow a man who was observing their response. He was the man that Hetty feared was involved, Kelvin Griffin.
He doesn't know the actual target location. In order to save Hetty's job, the NCIS:LA team goes on a rogue mission to retrieve the device from the Russian consulate. They succeed, but suffer the wrath of Granger.