The CDC team continue to investigate the island; at the same time, they discover Peter's brother, Alan, is a member of Brother Michael's cult, and his warnings for them to leave the abbey fall on deaf ears.

Michael struggles to keep the abbey under control as the CDC team move closer toward tracking the source of the pathogen; at the same time, Alan fights the urge to rejoin his former colleagues and comes to terms with the reality about Peter's allegiances. In the future, Walker takes a risk attempting to escape her delusional father.