On a frozen Morning in Montana, voices rise from a Forest Service cabin where 11 ice climbers warm their hands on bowls of oatmeal. It's early, the sky bright, the canyon still cold and shady. They're getting ready to clean up and head out, and they notice the absence of the 12th.

"Did Demond get food?" one of them asks.

"Dude should eat." "I think he's over by the fire."

Fifty yards through the cold, Demond Mullins sits in a snow crater watching embers from last night's fire flicker back to life. A band of light spreads down the peaks above him. Snow floats off a tree and comes to rest on his sticky-wet eyelashes. It melts outward into gobs of water, held together by each tiny hair. He stokes the fire with a piece of Douglas fir.

Mullins watches the fire for a long time. "There is never silence where I live. There is always someone in the apartment upstairs, footsteps," he says. Here in the canyon, his mind is as quiet as the snow, until he hears his name in the air—a flash of sound that dies immediately. He trudges toward the cabin.

The door opens with a creak. There is a change when he enters, subtle but sure. As he glides by the oatmeal buffet, climbers reorganize themselves around him, shuffling a couple of inches or tilting a few degrees. Often the most handsome and athletic person in a room, Mullins can shift the atmosphere with his mood. He overhears two climbers talking about the skateboards they rode as kids and suddenly bubbles over, laughing and asking questions. He tells a story about how he likes to get on his board and let his dog pull him around Brooklyn.

After a few minutes, the skateboarding conversation slows into silence, and Mullins's enthusiasm gives way to an aloof pensiveness. Spooning oatmeal, he merely smiles when people say, "Good morning," or "How are you?" Now 31, he has been an orphan, a dancer, a model, a soldier in the Iraq War, the subject of a documentary, and a Ph.D. student—in that order. In the next six months he will become a husband and a professor.

"Let's get ready to go," someone says.

Like everyone else here, Mullins is a veteran of the post-9/11 wars. When he returned to New York from Iraq in 2004, the city of his youth had warped into a collection of imagined threats—the civilians reaching into their coat pockets, the muffled sounds through the wall, the dark cars that seemed to follow him home from the subway. This trip, organized by the Sierra Club Outdoors program, is designed to give him tranquility and structure, respite from the urban clamor. It's also designed to teach him a new adrenaline sport, something that matches the exhilaration of combat but is viable for a life without weaponry or body armor.

Two days ago, Mullins flew to Bozeman, where he met the rest of the group at a gear shakedown in the basement of a hotel. The room was full of boxes and rubber tubs containing donated equipment for the novice climbers. The contents soon overflowed into tangles of harnesses, clumps of long underwear, and piles of gloves, sleeping bags, and shiny, ultrawarm coats. The four guides walked around to see that the participants, who were already bonding over their collective lack of winter-camping savvy, packed the right combination of stuff.

"I have two lightweight tops, a midweight top, plus these," said Mark, a former Army Ranger from New York City with a voice like a coffee grinder, who had laid three jackets of varying puffiness at his feet. "I'm swimming in these pants," said Josh, a former Army major from Tacoma, Washington, who, at five feet eight, was trying on an extra-large snow bib.

One climber went up to another and said with genuine concern, "You're wearing a women's helmet. Take this one instead."

Near the room's center, Mullins pulled some clothes from the boxes and slipped them on without hesitating, looking every bit the former model. But then he was thwarted by an ill-fitting glove.

"You want three pair of gloves. One for climbing, one for when those get wet, and one for when those get wet," a guide called out.

Mullins raised his gloved hand and repeatedly squeezed it into a fist. "How much space should I have in my fingers?" "Here," Mark said, raising his palm. They matched hands and decided that Mullins's long fingers should have the large gloves.

Amid all this, the group made small talk. It was mostly an inquisitive discussion of everyone's origins and current homes, with an undercurrent of aspiration. They discussed the topography of Indiana, the abundance of bluegrass concerts in Denver, the noise of New York, and the lurking smog over Salt Lake City. They seemed on the constant cusp of something. Many hoped to go to school, or they were in school and trying to pay for the next semester, or they were talking about starting businesses. In every case, they looked for clues in one another, eager to learn from other veterans' postwar successes.

Mullins stands at the base of a waterfall, roped in and ready to—as one guide put it—"beat the crap out of some ice."

The belayer checks Mullins first. "Five pairs of rope. Knot looks good. Harness looks good. Is that it?" "Nice," Sam Magro, a local guide, says. "Now check him, Demond." "Carabiner locked. Harness is good. OK. I think we're ready?" They both look at Magro. "All right. You guys are set. Let's do some climbing!"

With his crampons and ice tools (which have curved handles for climbing—unlike ice axes, which have straight handles for walking overland on ice), Mullins fastens himself to the frozen waterfall.