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.

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.

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

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.

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.

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

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.