Winslow and Rosie have been building an impressive collection of natural wonders, and now it's nearly complete. With just enough space left for one final piece, the girls set off into the forest to find something extraordinary. Their journey takes them far - along the creek, across the valley, through the spruce grove and up the mountain. And while they find many extraordinary things, nothing is just the right extraordinary thing. When the girls find a deep, dark cave, they decide to venture farther than ever before. Surely, a mysterious cave must hold something extraordinary...