- [Narrator] Esau has crossed back to Kenya.

He's only 33, but already his future is fading.

His wound is severely infected.

(plane engine whirring) Perhaps he's returned to find help.

Is it possible he knows that desperate measures are underway to save his life?

(conservation workers speaking in foreign language) (vehicle engine roaring) (dart fires) (conservation workers speaking in foreign language) - [Narrator] Esau faces much more than threat from infection.

A satellite collar, evidence of a scientific study might be his only hope of surviving trophy hunters.

These magnificent tusks are both a blessing and a curse.

When Esau awakens, his world will have shifted.

(cicadas trilling) (Esau huffing) (cicadas trilling) (grass rustling) (cicadas trilling) (crickets chirping) With an unfamiliar weight around his neck, Esau heads back across the invisible line.

Losing Esau would ripple far beyond a single life.

It would sever an ancient lineage that is irreplaceable.

Esau has crossed this border for decades, as have thousands of elephants, but the rules have changed.

To survive, elephants must be able to migrate.