When people start mysteriously dying in Los Angeles, Don and his FBI team are called in by the Public Health Service to determine if it is an act of bio-terrorism. Applying Charlie's knowledge of the mathematics of diseases, they track the virus not only to stop it, but also to find its point of origin. Once they determine who is responsible and why, they are surprised by the suspect’s motive. Meanwhile, Don is surprised to learn that Charlie has been keeping a secret from him. CCH Pounder guest stars.
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