A warehouse with five dead bodies, two of whom were apparently Marines, draws the attention of NCIS when a TV crew following ATF agents on a raid happens upon them.
The "Marines" were wearing authentic uniforms, but weren't actually Marines, so NCIS wants to oversee the investigation to make sure there's no military connection.
However, when the cause of death is determined to be Botulinum poisoning, the Los Angeles team takes the case because Botulinum is a deadly neurotoxin and bio-weapon.
Sam and Callen pursue their first lead, an elderly woman suffering from Alzheimer's, but it turns out she was a former military chemist and she was being manipulated.
Though her disease makes her an unreliable witness, the memory of her job has not been destroyed and she successfully produces enough biotoxin to kill millions of people.
Callen recognized one of the dead bodies as a former security guard for Dallas (from "Breach," the strip club owner with high-powered connections) and they track him down.
The team then discovers that a weapons dealer named "The Dutchman" is holding an auction for the poison - starting at $1 million. Sam goes undercover as a buyer.
There, he discovers there's going to be a "demonstration" at the Beverly Center. Sam's cover is blown but he successfully takes down everybody, while, Callen and Kensi race to the location and manage to thwart the attack.
They do, in the nick of time. Callen is exposed to the toxin but Sam arrives quickly with the antidote. Is there anything Sam can't do?