When Spinner McPherson, accompanied by Reglan, a friend, and Otter, his dog, hike deep into Echo Valley, the last thing he has on his mind that day is discovering a place where creatures long thought to be extinct still exist. But that's exactly what he and Reglan do. They discover Underland. And it's like no other place on or under the earth.
Spinner thinks 'his' discovery is the greatest of all time and immediately visualizes the fame and fortune that will come his way by reporting its existence to the outside world. Reglan views things differently. But, as Spinner succinctly puts it, she's a girl. So they part ways, each doing what they think is right.
Underland's discovery and its mystical secret propel Spinner on a journey that will affect him his entire life. He almost understands too late the true meaning of Underland and it becomes a race against time to prevent Underland and all its interesting and wonderful inhabitants from vanishing from the face of the earth forever.