IT didn’t take Anna any time at all to discover that running in snowshoes sucked. They caught in the rocks, they caught in the brush, they brought her to her knees twice, and only Charles’s hand on her elbow kept her from falling all the way down the mountainside. Jumping downed trees was . . . interestingly difficult. However, Charles, without snowshoes, was sinking up to his knees and deeper with each step—so she was properly grateful for hers.

That’s not to say they were slow. It amazed Anna what terror could do for her speed. After the first, terrifying sprint-slide down the steep slope they’d spent hours climbing, she...