Joe Simpson and his climbing partner, Simon Yates, had just reached the top of a 21,000-foot peak in the Andes when disaster struck. Simpson plunged off the vertical face of an ice ledge, breaking his leg. In the hours that followed, darkness fell and a blizzard raged as Yates tried to lower his friend to safety. Finally, Yates was forced to cut the rope, moments before he would have been pulled to his own death.

The next three days were an impossibly grueling ordeal for both men. Yates, certain that Simpson was dead, returned to base camp consumed with grief and guilt over...