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.

Sight-to-Summit Gallery

To view an exclusive gallery of the expedition, click here.

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."

There was something in it for everybody: The athletes would get to climb; The North Face would advance its philanthropic mission; the filmmakers would roll out high-quality footage; and several hundred Nepalis would have their sight restored.

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?"

It was 7:30, almost dark, by the time Geoff started in on his 21st consecutive, and final, operation. He'd been at it for seven solid hours. No food. No water. No bathroom breaks. Johnny Cash piped in from an iPod plugged into portable speakers on a small desk, where Geoff's 18-year-old stepdaughter, Ali Demarchis-Tabin, logged patient data into a laptop. (Geoff and his ophthalmologist wife, Jean, have a brood of five, including three kids from Jean's first marriage.) Once again, Geoff reached the crux of the procedure: coaxing the lens through the small incision, a move about as easy as extracting the yolk from a hard-boiled egg without removing the shell. After a few tense minutes, the lens slid out cleanly; Geoff nonchalantly brushed away the wasted scrap of tissue.

The nurse handed him a small blue case, which contained a replacement interocular lens, or IOL, produced at a lab in Kathmandu. Using tweezers, Geoff carefully removed the tiny, pliable plastic disc. It was shaped like a miniature galaxy, with two curved spurs, called haptics, sticking out from opposite sides to secure it in place. He slipped it through the incision, which would seal without stitches, and leaned back from the table, his latexed hands floating in the air like a conductor's.

"This morning she saw only light and dark," Geoff said.

"Tomorrow she'll have 20/20.

"Take a look," he added, nodding down at the microscope. "It's perfect."