Near the forks of the Grand River in present-day Perkins County, in August 1823, while scouting alone for game for the expedition's larder, Glass surprised a grizzly bear mother with her two cubs. Before he could fire his rifle, the bear charged, picked him up, and threw him to the ground. Glass got up, grappled for his knife, and fought back, stabbing the animal repeatedly as the grizzly raked him time and again with her claws.
Glass managed to kill the bear with help from his trapping partners, Fitzgerald and Bridger, but was left badly mauled and unable to walk. When Glass lost consciousness, Henry became convinced the man would not survive his injuries.
Henry asked for two volunteers to stay with Glass until he died, and then bury him. Bridger (then 17 years old) and Fitzgerald stepped forward, and as the rest of the party moved on, began digging his grave. Later claiming that they were interrupted in the task by an attack by "Arikaree" Indians, the pair grabbed Glass's rifle, knife, and other equipment, and took flight.
Despite his injuries, Glass regained consciousness. He did so only to find himself abandoned, without weapons or equipment, suffering from a broken leg, the cuts on his back exposing bare ribs, and all his wounds festering. Glass lay mutilated and alone, more than 200 mi (320 km) from the nearest settlement at Fort Kiowa on the Missouri.
In one of the more remarkable treks known to history, Glass set his own leg, wrapped himself in the bear hide his companions had placed over him as a shroud, and began crawling. To prevent gangrene, Glass laid his wounded back on a rotting log and let the maggots eat the dead flesh.
Glass survived mostly on wild berries and roots. On one occasion he was able to drive two wolves from a downed bison calf, and feast on the meat. Reaching the Cheyenne, he fashioned a crude raft and floated down the river, navigating using the prominent Thunder Butte landmark. Aided by friendly natives who sewed a bear hide to his back to cover the exposed wounds, Glass eventually reached the safety of Fort Kiowa.