: G. Callen (Chris O'Donnell) infiltrates the group by staging a public brawl with Sam Hanna (LL Cool J). This leads to a tense sequence where Callen's cover is blown, and he is nearly killed by the very cyanide he's investigating, only saved by a timely counteragent.
Critics from TV Fanatic rated the episode a 4.2 / 5.0 , praising the strong team interaction and the focus on core cast members rather than outside guest stars. While some plot points were deemed predictable, the ideological conflict presented by the DNA group added a layer of complexity often missing from standard procedurals. NCIS: Los Angeles Season 4 Episode 20 Recap: Purity NCIS.Los.Angeles.S04E20.HDTV.x264-LOL.[VTV].mp4
: The episode features interesting trivia about "Prussian blue," a pigment used as an indicator in cyanide tests and a treatment for radiation poisoning. Critics from TV Fanatic rated the episode a 4