WHEN I FINALLY ARRIVE in the remote Nepal­ese village of Nar, the locals offer up many theories about how and why seven young Gorkha men were found dead just outside their tiny town in the summer of 2009. The chief’s brother suspects they slipped while traversing a steep mountain. A horse breeder thinks a boulder shook loose, trundled down a precipitous slope, and knocked them over a cliff. A teahouse owner claims the violent deaths are simply a mystery, the inevitable workings of nature and man in that isolated high dale that outsiders call the Lost Valley. “No one has any evidence of anything,” she says.

It was back in August 2010 that I first heard about the deaths. A short Agence France-Presse story reported that the men had been murdered, the result of a dispute over yarcha­gumba, a rare and highly prized fungus. The Narpa, as the people of Nar are called, were picking the fungus in their home fields near Tibet when they ran across yarchagumba poachers from the fierce Gorkha tribe to the east. A couple weeks later, the district ­police discovered two Gorkha men at the bottom of a steep escarpment; the other bodies were rumored to have been cut into pieces and thrown in a nearby river, but they were never found. Suspecting the inhabitants of Nar, the police arrested 70 men, nearly the village’s entire male population.

Yarchagumba looks like a shriveled brown chile pepper and is coveted as an aphrodisiac and medicinal cure-all. Literally translated as “summer grass, winter worm,” it forms when a parasitic fungus invades the burrowing larva of a ghost moth, transforms the ­vital ­organs into a cobweb-like mess, and then sends up a wispy sprout through the dead ­insect’s head. The grisly process plays out across the Himalayas and the ­Tibetan Plateau but only at the beginning of the monsoon and only on reclining slopes of grasses, shrubs, and milk vetch at the dizzying altitude of 10,000 to 16,500 feet. Thanks to a spike in global demand, mostly by Asian men looking to enhance their virility, a pound of yarcha­gumba now sells for as much as $50,000, more than the price of gold. ­Profits from the fungus have transformed entire ­villages, vexed government regulators, and even helped bankroll a communist insurgency. Nepal’s former Maoist rebels admit that taxing (read: ­extorting) yarchagumba pickers was their main source of income in their decade-long war against the country’s monarchy.

I couldn’t believe what I was reading. I had visited Nepal often over the previous 15 years, even lived there for nearly a year, and had never heard of it. But as soon as I read about what the tittering international press dubbed the “Himalayan Viagra murders,” yarchagumba began popping up everywhere.

In a Chinatown apothecary near my apartment in New York City, I saw the caterpillar fungus for sale in tall glass jars, under the Chinese name dongchong xiacao. Then, in July, two goons wielding bear spray robbed fistfuls of the stuff from a holistic-healing center in Richmond, British Columbia. ­Every Nepali friend in my neighborhood, from the hardware-store clerk to the bodega ­manager, had at least heard of yarchagumba, if not sampled the prized herb. When the checker at my grocery store turned out to be from Phu, Nar’s equally tiny sister village, the coin­cidence was too serendipitous. I had to go.

Though it was now autumn 2010, the timing of my visit was opportune. Thirty-four of the men arrested in Nar had been released, but 36 had been charged with murder and, astoundingly, more than a year and a half ­after their arrests, were still awaiting trial in a make­shift jail downvalley in Chame, the district’s head­quarters. After many delays, the legal proceedings had recently been rescheduled for early November. If I hustled, I might be able to nose around Kathmandu, trek to Chame to witness the verdict, and then continue on into the Lost Valley for a little fact-checking.

What I would discover after all that—two weeks in Kathmandu, five days of trekking, and three days in Nar—was a mystery as bizarre and unpredictable as it was difficult to unravel.

I LANDED IN KATHMANDU in mid-October. The sunlight of the Himalayas, always sharp in the dry air of fall, appeared to shine with particular clarity, delineating each receding layer of the city’s jumbled sprawl—the ­cement homes, the bulbous spires, the riot of birds overhead.