This book is about Circle. This book is also about Circle's friends, Triangle and Square. Also it is about a rule that Circle makes, and how she has to rescue Triangle when he breaks that rule. Triangle and Square are visiting Circle, who lives at the waterfall. When they play hide-and-seek, Circle tells her friends the one rule: not to go behind the falling water. But after she closes her eyes to count to ten, of course that's exactly where Triangle goes. Will Circle find Triangle? And what OTHER shapes might be lurking back there?