His world shaken, Gus runs off and makes a rash decision. Big Man comes to grips with long-ago mistakes. Aimee makes a last stand against General Abbot.
Gus follows a reluctant Big Man into the unknown. Dr. Singh discovers what it will cost to save Rani. Aimee finds a new beginning after the world ends.
After a daring rescue, Gus meets an individual well-versed in hybrid history. Amid emotional reunions and eerie revelations, all roads lead to the cave.
With loved ones beside him, Gus goes on a treacherous journey into the cave to find a cure once and for all. Meanwhile, chaos breaks out at the outpost.