When a new type of chemical warfare is released into the open air, the BAU must act quickly to find the source before any more innocent people are exposed.
The team is called to Montana to investigate the case of four kidnapped and murdered women. When another woman disappears, the search leads to a man who apparently has assumed the identity of his dead partner.
The BAU investigates a series of murders that appear to be connected to a killer who jumps trains and chooses his victims based on their proximity to the railway.
The Man informs the Messengers one of their own has betrayed them and another is in imminent danger, while Amy's drawings predict the arrival of someone who could change the course of everyone's destinies.
The BAU travels to Houston to investigate a series of murders that have occurred near construction sites and abandoned buildings. Because the murders appear to be random, the team believes the killer could be a homeless person. Meanwhile, everyone is noticing an apparent change in Reid's behavior.
Gus' expedition with Dr. Singh leads to several shocking discoveries. Big Man and Aimee seek refuge and reinforcements. Becky connects with a new friend.
After an eventful, obstacle-strewn journey to find a safe haven, Gus learns more about his origins and how far Birdie was willing to go to protect him.
On the hunt for the Horseman of War, Joshua, Erin and Peter make great strides for the group. Raul and Vera are more hesitant to believe and when they stray, there are unforeseen consequences. But when Vera discovers her gift, she finally starts to rethink her purpose.
A captive Nora is tortured and left for dead. Monroe learns of Rachael’s attempt to reach the Tower but is unaware of her deadly plan for him. Meanwhile, Miles is desperate to find out how so many of his close associates are being brutally murdered. Later, Miles, Charlie, Neville, Jason, Hudson and Sanborn struggle to make it out of the Plains Nation to join Rachel and save her from what they believe to be certain death.
"100" is the best rated episode written by Oanh Ly. It scored 8.8/10 based on 17 votes. It was directed by N/A. It aired on 11/25/2009 and is rated 0.2 points higher than their second-best episode, "Amplification".