The BAU heads to Los Angeles, California, where a sarin attack on a CityLink bus has left all ten inside the bus dead. A homemade timing device found on the bus was what was used to release the sarin without the unsub needing to be on the bus. The team looks into any chemists who may have had access to sarin stockpiles and had a grudge of some sort as a possible suspect. But a man named Charlie Senerak jumps to the top of the suspect list when surveillance footage shows that Charlie was sitting in his car in a park and ride lot where the bus originated, and that he and his wife Tracy, with who he works, usually take that bus, but neither who were on it this day. In questioning Charlie, Rossi and Lewis don't believe Charlie to be the unsub, but they do know that he is hiding something major that he did not want to divulge to them, meaning that that something could be related to the attack. As further evidence piles up on Charlie, the team, if they discover the truth, will find that Charlie is indeed involved peripherally, the unsub who is blackmailing Charlie to carry out of deadlier attack, with the questions being what that blackmail issue is, why the unsub chose Charlie, and how the unsub is able to manipulate Charlie at every step along the way.