On the tropical island of Banana, Princess Banna and her loyal servant extend an invitation to Anpanman and his companions for their grand celebration known as "Banana Matsuri". However, when they arrive on the day of the festival, a sudden storm and chilly weather engulf the island, causing all the bananas to wither away. The princess and Banna are frantic in trying to find out what caused this calamity. They decide to investigate further up Banana Mountain. Suddenly, a water monster emerges from the depths of the sea threatening to take the entire island back into its aquatic realm. With Banna's assistance, Anpanman and his friends embark on an urgent mission to save Banana Island from this impending disaster.