Thirteen-year-old Colter's summer vacation takes a twist when he finds himself on a broiling city sidewalk, on the hottest day of the year. The young man finds hope in an ad in the back of a comic book. It promises to send a special rock, for free, that he can plant in his backyard. The comic claims that rock will grow into a massive mountain almost no one else can see with rushing streams, towering evergreen forests, and lofty granite peaks covered with snow.

Colter thinks it sounds like a scam but curiosity makes him send for it anyway. Imagine his surprise when everything the ad promised comes true! Time stands still in the ordinary world while he explores his own majestic wilderness. There he meets a mountain man named Bridger, who explains the ways of the wild. He warns Colter of the many dangers that lie ahead. He must seek the help of skilled climbers from a local mountaineering lodge, including a girl his age named Chrysanthemum.

Will they be able to overcome the challenges of convincing parents that can't see the mountain? They also must face off against ferocious grizzly bears, towering cliffs, and dangerous weather to find a route to the top of Colter's Mountain. Worst of all, can they escape the deadly plot of a sinister enemy known as the Alpinist?