What makes a fish a fish? Trout thinks he knows the answer. "That's easy! Fish have fins, gills, scales, are shaped like me, and live underwater." "Not so fast," answers Mudskipper. "What about a clingfish? They don't have any scales. Or a hagfish? They don't have any fins. Or what about me? I live in and out of the water!" As Trout and Mudskipper explore below and above the world's waters, they are introduced to a multitude of fish in various shapes, colors, and sizes, forcing Trout to rethink his notion of what a fish is. Maybe there is a vast watery world of unimagined possibilities (like a walking fish, or a fish with a transparent head!). And maybe, just maybe, there's not just one way to be a fish--but many, many ways!