Harry the dog likes the seashore, except the heat. Harry's human family likes everything about Harry, except his tendency to crowd their beach umbrella and wreck their sandcastles.
As Harry sits alone by the water's edge, a big wave crashes over him, leaving him covered with seaweed and looking like "something from the bottom of the sea." Now Harry is cool and comfortable--and completely oblivious to the terrifying sight he presents as he runs from one identical striped beach umbrella to another, searching for his family.
Just as Harry is about to be captured and taken to the aquarium as a Bushy-backed Sea Slug, everyone is happily reunited at a hot-dog stand. For their next trip to the seaside, Harry's family buys a brand-new beach umbrella: big enough to cover everyone--including Harry--and easy for Harry to find, because, like him, it's white with black spots.