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.

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.

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.

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.

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?