Growing up in a remote forest cabin with his dad, Gus learns lessons about survival and the dangers that lurk beyond the fence to the outside world.

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.

In a moment of desperation, Dr. Singh makes a bold promise. A lost memento makes for a bumpy ride to Colorado. The Last Men close in on the Preserve.

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.

Locked up in the zoo's kennels, Gus meets his fellow hybrid prisoners as he looks for a way out. General Abbot pressures Dr. Singh to produce a cure.

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.

After suffering an unimaginable loss, Gus must make a choice that will determine the fate of humans and hybrids. Who will be left to tell their story?