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.

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 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.

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.

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.