With temperatures plunging to –40 °C and several metres of snowfall, Yellowstone freezes solid for six months each year. In the extreme cold, moisture in the air freezes, creating diamond dust. The severe winter is the greatest challenge facing the Park's animals, but for the wolf, it is the season of opportunity. The film follows the Druid pack, one of the largest in Yellowstone, as they stalk ever-weakening prey. As herds of elk move to sheltered valleys at the edge of the Park, the wolves lie in wait. Other scavengers such as coyotes and bald eagles also take advantage of wolf kills. On the open plateau, bison are built to endure the worst of the winter, bulldozing their way through deep snow to reach grass. In the harshest winter for a decade, even they are forced to move, following a thermal river to a geyser field. The grass here is laced with silica and arsenic – if the bison stay too long, it will poison them. Aerial shots reveal the tracks of otters and coyotes moving through the snow. The coyote steals a trout stashed beneath the ice of Yellowstone Lake by the otters. The red fox is another year-round resident, listening for and pouncing on mice moving under six feet of snow. By the end of March, winter has loosened its grip, and the emergence of grizzly bears from hibernation in their mountain dens signals the approach of spring.

As the Sun gains strength,one direction the Park begins to thaw and grazers move back up to higher altitudes. The hardy bison are amongst the first, their newborn calves struggling to cross rivers swollen by meltwater during the journey. Bison, along with elk, pronghorn and migrant birds, are drawn to the fertile pastures of the plateau. Newborn pronghorn twins must use the long grass for camouflage against prowling coyotes. There is a rare sight of wolf pups as the Druid pack venture from the woods to visit the riverside. By June, the blooming wildflower meadows attract calliope and rufous hummingbirds, but when a summer blizzard blows in, the birds must turn to sugar-rich tree sap to survive. As summer returns, a new challenge emerges: Yellowstone begins to dry out. Yellowstone cutthroat trout mass in shallow streams ready to spawn, but they make easy prey for otters and osprey. In August, the hot, dry plateau is the scene of the annual bison rut. Lightning can spark fierce wildfires, but the short-term devastation is part of the ecological cycle and helps the lodgepole pine forests regenerate. Meanwhile, grizzly bears converge on the high mountain slopes as they seek out an unusual food source: army cutworm moths, which arrive in their millions from the prairies. A female bear is injured in a clash with a male, but they must take advantage of the bounty before the brief summer is over.

Autumn is Yellowstone's shortest season and a period of swift change. Conditions change from summer to winter in just two months, forcing animals to leave or prepare for winter. Bison rely on stored fat to see them through, but elk and pronghorn head for lower ground. Their only natural enemy is the wolf, but beyond the Park boundaries they must contend with different hazards: hunting, heavy industry and traffic. Since their reintroduction to Yellowstone in the 1990s, wolves have expanded their range, bringing them into contact with people. Telemetry enables ranchers to track wolves fitted with radio collars and scare them off. The wolf's return has restored the natural balance of Yellowstone: elk no longer graze along the river banks, leaving more willow saplings for beavers. In autumn, beavers are busy repairing their dams and harvesting saplings for their underwater larder. In the Beartooth Mountains, pine squirrels, grizzlies and Clark's nutcrackers take advantage of a bumper crop of cones from the whitebark pines. The trees are under attack from beetle larvae, which now survive through the winter due to the warming climate. By late September, bull elk are exhausted from six weeks of rutting and herding. As the females come into season, only those bulls who still have the strength to see off challengers will have the chance to mate. In Yellowstone's mountains, male bighorn sheep clash horns as the first snows of winter arrive.