As an imminent flu epidemic threatens the U.S., Federal Agent Brad Wolgast is tasked with bringing in the experiment's latest chosen test subject, a ten-year-old girl, Amy Bellafonte. But, after meeting Amy, Brad's loyalties to Project NOAH are tested.

Amy and a wounded Brad continue on the run from Richards and attempt to seek refuge with an elusive, ex-military instructor of Brad's, while Lila tries to help from afar. Meanwhile, at Project NOAH, Dr. Lear reflects on the decisions that compelled him to involve Fanning in his research, and testing of the virus on Anthony Carter begins, as the epidemic in China spreads and the race for a cure intensifies.

Brad strikes a deal with Sykes and Richards to stay with Amy at Project NOAH, as they begin to test the virus on her. Upon learning the harsh truth regarding the state of inmates in 4B, Brad forms an alliance to get more information. Meanwhile, Babcock sheds light on her past.

In an attempt to protect Amy, Wolgast convinces Sykes and Lear to let them outside of the facility for exercise. As emotional flashbacks reveal Carter's life leading up to his death-row sentencing, Fanning's hold on those kept inside Project Noah grows stronger. Meanwhile, the virals gain power over the facility's staff, leading Richards and Wolgast to reunite so they can take down a rogue soldier.

With the sudden arrival of a high ranking official, Horace Guilder, to Project NOAH, Brad and Amy begin to strategize a possible escape plan with the help of an unlikely accomplice. Meanwhile, Sykes and Richards begin to question each other's motives, and a shaky past between Lear, Elizabeth and Fanning is revealed.