The new season will focus on the 2001 anthrax attacks in the United States, a biological terrorist attack that occurred in the United States for several weeks starting on September 18, 2001. Letters containing anthrax were sent to several news media offices and two Democratic senators starting on September 18, 2001. The incident resulted in five deaths and 17 infections. The main suspect was not announced until 2008. Kim Dae-hyun plays an FBI agent who specializes in microbiology, and Goldwyn plays an Army biologist, whose role is based on the experience of the real person Bruce Ivins.