Each year more than a million wildebeest, joined by hundreds of thousands of zebra and gazelle, move in a single circuit across the Serengeti, crossing from Tanzania into Kenya's Masai Mara and back again in search of fresh grass. The film follows the herds through the cycle: the calving season on the southern plains, the long march north, and the river crossings at the Mara and Grumeti, where crocodiles wait in the current and lions and hyenas work the banks. Cameras track individual animals through the chaos of a crossing, capturing the drowning, the stampedes, and the calves that get separated from their mothers. Weather and water levels dictate the route year to year, and the film treats the migration as a single connected system rather than a series of isolated set pieces. The scale is the point: this is one of the last mass movements of large mammals left on Earth.