Twilight Sparkle and her friends have been invited to the Kingdom of Windermare to celebrate the return voyage of Prince Admiralty and for the rulers of Windermare to pay their respects to the newest princess of Equestria. But what was originally expected to be a simple meet-and-greet turned into a mission to keep Windermare's King Azimuth and the Chieftain Nautilus of the sea ponies of Equarius from declaring war on each other. To make matters worse, a sea witch called The Kelpie is attempting to thwart the ponies' efforts to keep the peace. But during this time of uncertainty, Fluttershy finds herself drawing closer to Prince Admiralty, and Twilight finds a powerful ally in the Prince's old mentor, Epoch. When an Equarian royal goes missing, it's up to the Mane Six, the prince, Epoch, and the prince's friend Captain Seaworthy to solve the mystery before Chieftain Nautilus brings war to the Kingdom of Windermare.