Shelly is tired of her brother, Gordy. She'd much prefer a sister. So when Coral catches a wave to shore, they are instant friends, even if one of them has flippers instead of feet. They dance and play and eat peanut butter and jellyfish sandwiches. It's great having a mermaid sister! But when Shelly and Coral get in their first fight, Shelly wonders if Coral might be happier back at sea. Life with a sister can have its ups and downs, but it's nothing another day at the beach can't fix.