One summer, young Ronnie's father puts a wading pool in the backyard. Ronnie loves playing in the water and even tells his mom he's going to be a deep-sea diver someday. Following a heavy rainstorm, the wading pool takes up most of Ronnie's backyard—which is when something amazing happens. A colorful fish swims by. Ronnie thinks the fish might just continue on its way; instead, Mr. Fish invites Ronnie on an adventure to visit an underwater city. These two new friends swim along together and see all sorts of interesting creatures, including a parrotfish, butterfly fish, triggerfish, and more. Ronnie's day culminates with a parade of extraordinary colors. Fish swim, dance, and even play music while Ronnie learns what makes each of them special and unique. Eventually, Ronnie has to return home ... but his day of surprises isn't over yet! A celebration waits, and Ronnie will soon have even more fish friends.