Mermaid sisters Cassandra and Alexandra are about to celebrate their 13th birthday. They are filled with excitement as this milestone birthday comes with a very special gift, a chance to see the world above the ocean. Their friends, Diana the dolphin and Willy the whale, are worried that they are too young and adventurous for such an important experience. However, all mermaids are allowed to go on this adventure for their 13th birthday, and these mermaid sisters were not going to miss out. So, with strict instructions from their father, the girls started getting ready for the adventure of a lifetime! Little did these excited mermaids know, this birthday present was about to change their lives forever. It would also leave a lasting impression on one little boy, who would never forget the two beautiful mermaids on the rock.