Peter Farrugut, Sarah Jordan, and Dr. Kyle Sommer go to a mysterious island to investigate an unknown pathogen.
Jordan discovers that Peter's brother is a member of Brother Michael's cult; Jordan and Peter refuse to leave the Abbey to investigate Soren's infection.
When Peter and Jordan leave the Abbey to retrieve specimens, Sister Amy plots to make their leave permanent.
Michael tries to maintain control over his sisters; Alan is suspicious of Peter's true motives for being there.
Peter reveals his brother's secret, getting them both into trouble; Kyle tries to quarantine the orchard and the infected hive.
The CDC team must work together when another outbreak starts to overwhelm the abbey.
Walker decides to go to St. Germain when he learns about Michael; things go from bad to worse on the island.
The Coast Guard takes the CDC team to another island with an outbreak; Sister Amy must contend with Brother Michael and his plans.
Alan, Kyle and Winger search the woods for the tree with red sap which may contain the cure for the pathogen.
Sister Amy makes a bargain for immortality; with Kyle infected, the race is on to find the Bleeding Tree.
The Navy prepares to implement Plan B, which will ensure the virus and the infected people do not leave the island.
Peter finds his purpose as the new leader of Michael's order as the team continues the desperate search for the Mother Tree.
Walker and Alan's relationship comes to a bloody climax; Peter and Alan's sibling feud erupts in flames; Kyle tries to protect a possible mycosis cure; Amy plots against Jordan and Walker.