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?

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.

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 and his cohorts set out for Alaska to reconnect with Birdie, who's working on a cure for the Sick. But first, they test their luck at an old casino.

After a doctor joins Gus and his crew, they seek a way to the coast and stumble upon a plane owned by a family in desperate need of medical attention.

An unsettling discovery at sea forces Gus to take on a grim chore as tensions swell between his companions. On land, an enemy interrogates Becky.

As polar night looms, Gus and his crew arrive at the outpost — but they're not alone. To find Birdie and the cave, they must brave several obstacles.

Gus's quest to find Birdie and uncover more about her connection to his father leaves him questioning everything he's ever known.

As Gus learns more about a threat and their dangerous plans, the group races to catch a departing boat. Jepperd and Becky face an impossible decision.

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.

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.

Eager to send Gus on his way, Big Man figures out how to hide him in plain sight. The Singhs attend a neighborhood party that takes an ominous turn.

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.

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.

An uneasy alliance forms over plans to free Gus and the other kids from captivity. Desperate for a breakthrough, Dr. Singh conducts a risky experiment.

As Big Man reckons with sins of the past, Gus becomes a target of anger and suspicion, Rani's misgivings multiply, and Becky begins basic training.

The zoo turns into a battle zone when the rescue mission takes flight, with families coming together — and falling apart — during the chaos.

Gus and Big Man pick up a traveling companion who suggests a perilous shortcut. Aimee's sanctuary comes under threat. Time's running out for the Singhs.

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.

Gus lands under the protection of some new friends as Big Man's past catches up to him. Dr. Singh fears a nosy neighbor will unearth Rani's secret.

General Abbot makes an example out of Gus. Dr. Singh obsesses over his research ahead of an important presentation. Becky crosses paths with a frenemy.

After an eventful, obstacle-strewn journey to find a safe haven, Gus learns more about his origins and how far Birdie was willing to go to protect him.

Warned about General Abbot's plans, Gus and his friends prepare to take a stand and defend the hybrid kids, no matter what it takes.

Gus' expedition with Dr. Singh leads to several shocking discoveries. Big Man and Aimee seek refuge and reinforcements. Becky connects with a new friend.