Set in a mysterious city on water, a baffling maze of canals and alleyways provides the glamorous setting for the continuing adventures of Ash and his Pokémon who travel to the Water Capital of the world, Alto Mare, where they meet two new Pokemons, a brother and sister pair of birds named Latias and Latios. The birds protect a treasure called the Droplet of the Heart, which is the target of a pair of thieves, Zanna and Rion. When the mysterious city accidentally floods, the treasure becomes endangered--can Ash save the day?