During the seventh night of an eight-day ascent up Denali’s demanding Cassin Ridge, Mike Helms and I anchored our tent to a precarious perch at 18,000 feet. That night, we survived hurricane-force winds and in the morning, woke to temperatures that had plummeted to -35 degrees. We waited for the sun before starting for the summit, but the sun offered little warmth, and in the oxygen-deficient atmosphere we struggled to move one foot higher than the other. When we reached 19,500 feet, we saw a small tent and a single climber waving frantically. We arrived to find a man named Jack Roberts; his toes were frostbitten, and his partner, Simon McCartney, lay in the tent semiconscious with high-altitude cerebral edema. We considered our options and hatched a plan: Mike and Jack would continue over the summit and return with rescuers, and I would wait with Simon. They left, but the…