Nearly 2,000 years ago, a volcanic eruption in Alaska poisoned the rivers that sockeye salmon depended on, and this film follows the population that survived by returning to the open ocean and undertaking a 6,000-mile migration to keep their species alive. The camera tracks the salmon through the stages of that journey, from river spawning grounds through predator-filled coastal waters to the deep Pacific and back again, using underwater footage and expert narration to explain why each leg of the route is lethal. Marine biologists lay out the physiological changes the fish undergo as they move between fresh and salt water, and the toll those changes take. Predators appear at nearly every stage, from bears at river mouths to orcas offshore, and the film treats the migration as a numbers game where only a fraction of any generation completes the full circuit. The result is a close look at one of the longest and most hazardous migrations of any animal, framed around the geological disaster that forced it into existence in the first place.