When a big blue whale comes to live near their reef, there is a misunderstanding between him and Rainbow Fish and his friends that leaves everyone very unhappy and hungry. Rainbow Fish crosses paths with the big blue whale. The gentle creature loves watching the sparkling fish at play, but because of his size, the little fish are suspicious and assume he is up to no good. The misunderstanding quickly escalates as, hurt and angry, the whale lashes out at them. Their worst fears seem to be coming true, and it's up to Rainbow Fish to save them all from disaster.