When Twilight Sparkle receives an invitation to visit the underwater city of Aquastria, she invites all her pony friends and Spike to travel with her. The ponies are welcomed by the ruler of Aquastria, King Leo, who sits on the Cosmic Council alongside his cousin, Princess Celestia. There is a long-held rivalry between the seaponies and the mermares. The two groups compete each year in a race for the grand prize, which is a beautiful pearl. Join the ponies and Spike as they confront danger, make new friends and race to the finish in this shimmering new world!