On a warm mid-April day in northeastern Nepal, on the outskirts of the village of Phaplu, scores of Nepalese—all of them blind or partially blind—lined up at a screening table in the grassy courtyard of the Solu Regional Hospital. They had walked or been carried here by the hundreds—Brahmans, Chetris, Rai, Tamang, Sherpas, Newar, members of the many tribes and castes commingled in the countryside—to undergo a form of treatment that they considered not so much modern as mystical. Around them, radiant sunshine strafed the crenellated, 20,000-plus-foot peaks of Khatang and Karyolung; schoolkids skipped along the hardpan main street past a convoy of porters hunched under their loads; teenage soldiers from the Royal Nepalese Army idled by the airstrip in blousy blue fatigues, rifles slung low like guitars.

But the patients were oblivious to all that. They waited, clutching their pre-op paperwork and the hands of their relatives, looking nervous or bewildered, lost inside themselves.

In the operating room—its spare furnishings limited to a stainless-steel table, a sink, and some antique metal cabinets—49-year-old American ophthalmologist and mountaineer Dr. Geoff Tabin sat in a ripped vinyl office chair, hovering over his first case of the day. For the past 11 years, Geoff has run the Himalayan Cataract Project (HCP), a U.S.-based nonprofit that's raised more than $2.9 million to provide eye care in impoverished areas of Nepal, India, Bhutan, and Pakistan. The bulk of that money, $2.2 million, has gone to the Kathmandu-based Tilganga Eye Centre, whose surgeons have cured an astounding 74,903 cataract cases since 1994—including 34,070 at mobile eye camps like this clinic.

"This one's like a 5.12," Geoff said brightly, peering through his microscope into the milky cataract of a 76-year-old Nepali woman named Chandra Maya. "I'm going to take it real slow."

Slow isn't Geoff's usual pace. The athletic, five-foot-eight physician—director of the Department of International Ophthalmology at the University of Utah's John A. Moran Eye Center, in Salt Lake City—has so much energy that it pulses out of him in a nearly constant stream of ticks, twitches, and jokes. That energy has carried him up Mount Everest three times, among other peaks—hence his use of the climbing lingo. A less complicated surgery rates a 5.10. The simplest would get a 5.9, but that's as low as he'll go. Geoff is curing blindness, after all; it's never exactly a walk-up.

Geoff swabbed the woman's eye with a cotton ball soaked in Betadine, then propped her eyelid open with a speculum. The procedure required only local anesthesia, and the woman squirmed on the table. "No, no," a nurse trainee barked in Nepali. "Don't move."

Geoff hooked a needle through the rectus, the gossamer-thin muscle that controls eye movement, then immobilized it with a suture. Next he angled in with a narrow scalpel called a crescent knife, making a small incision along the side of the cornea and working his way carefully toward the cataract—the eye's opaque, calcified lens. The air was pungent and antiseptic, stoking my light-headedness as Geoff talked cheerily about his profession.

"Medicine is one of the great bastions of risk-averse overachievers," he said. "You're virtually assured money, a job, respect in the community. But how does that become meaningful? How does that make the world a better place?"