NCIS: Los Angeles Episode 4.20 Purity

When an exotic dancer and a Navy Lieutenant are killed in a bikini bar after ingesting water laced with cyanide, the NCIS:LA team is called in to find the people responsible for the poisonings. Initially, an Arabic blog post claiming responsibility for the poisonings suggest that it was the work of an Islamic terrorist group, but a second round of cyanide poisonings near a porn set in Chatsworth makes the team suspect that the terrorists are homegrown with different motives. When Eric and Nell learn that a container of cyanide big enough to kill thousands of people was stolen from a local LA facility, the team kicks the investigation into high gear and sends Callen undercover as a radical to infiltrate a group called Protecting the Future (PTF), which Eric linked to the poisonings. The team follows the evidence and eventually learns that the leader of PTF, a former Navy officer named Daniel Fryman, plans to poison the water supply in Downtown Los Angeles, believing that eradicating the criminals and drug addicts will make the streets safer for children in the future. Luckily, the NCIS:LA team manages to locate Fryman and his men, subdue them, and prevent them from poisoning the city's water supply.