Dr. Alan Farragut leads a C.D.C. team to a remote Arctic research base to investigate the outbreak of a virus unlike anything ever seen, which has claimed his brother Peter as its latest victim.
The hunt for Peter continues, while Sarah tries to contain a trio of possibly infected scientists in isolation. Meanwhile, Drs. Walker and Hatake replicate Peter's experiment and discover the apparent source of the outbreak.
Alan learns of the existence of a lower level, where he plans to create a viable isolation unit for the infected -- now numbering in the dozens. Julia struggles to recall her attack, and Dr. Boyle is convinced that Hatake is thwarting her progress. Meanwhile, Dr. Jordan develops a way to test for the virus.
Alan tries an experimental anti-viral treatment on his brother, with encouraging results. While Dr. Jordan comes to terms with her test's failure -- and her own secret ailment -- Walker fights to survive after being outcast in Level R.
The team discovers Doreen, prompting an investigation that leads Aerov to Balleseros -- who in the meantime has been ordered to locate a mysterious "Dr. Hvit." Hatake finds Julia and her new friend on Level R, and Sarah faces the prospect of her patient turning into a Vector.
Alan attempts to penetrate the sealed Level R to retrieve Dr. Walker, who is struggling with hallucinations as her condition worsens. Elsewhere Major Balleseros finds himself rescued by -- or the prisoner of -- a new player, and Hatake reveals the secret of the virus' intended purpose.
The arrival of a team from the Ilaria Corporation brings a ruthless woman who is, for all intents and purposes, Hatake's boss. Julia awakens to find that she has evidently been cured, but with frightening side-effects. Outside, Balleseros and Anana make their way to the base.
Sutton learns the truth about Dr. Walker and holds her prisoner, declaring her to be the property of Ilaria. Anana tries to convince Daniel to leave the base with her, and Alan and Sarah make a move against Sutton.
After learning that Hatake is storing the original Narvik virus in a secret level beneath the base, Farragut leads a carefully coordinated mission to destroy it -- but first must get past the Vectors and their new leader. Balleseros takes advantage of the situation to escape confinement.
Learning that the Narvik virus has been stolen Alan and Julia pursue the culprit to a remote satellite station, where they find another of Hatake's buried secrets. Meanwhile, Dr. Jordan fights to find a cure before her condition takes her life.
After Alan has the remaining survivors isolated, Peter leads the Vectors in an attack plan to spread the virus. Meanwhile the CDC team devises a plan to cure the Vectors one by one, while an army of Ilaria mercenaries close in on the base.
Alan and Hatake set a trap for the assassins sent by Ilaria to retrieve both the virus and the cure. The Scythe learns the fate of Constance Sutton, and Daniel makes a fateful decision.
While the team fights against the Scythe and attempts to rescue his surprise prisoner, Alan and the others learn of an outbreak of the virus in Puerto Rico. Sergio returns to the base with one final mission.
