It is the year 2301, one hundred fifty years after terraforming operations commenced on the planet Aqua—formerly Mars—leaving it almost entirely submerged in water. Aqua's capital, Neo-Venezia, is a near-perfect replica of the wonderful, dreamy city of Venice on Manhome—previously known as Earth. Both citizens and tourists take their time exploring the city and admiring its rustic charm, particularly by riding one of the many gondolas gliding along the waterways. Highly skilled and knowledgeable women known as Undines navigate the gondolas and serve as tour guides for Neo-Venezia. Originating from Manhome, several well-renowned Undine companies have established themselves in the capital, with many young girls aspiring to join their ranks. Pursuing her dream of becoming an Undine, Manhome native Akari Mizunashi begins her training as an apprentice at the Aria Company. Shortly after arriving in town, she becomes acquainted with her mentor Alicia Florence, a pudgy cat named Aria Pokoteng, and Aika S. Granzchesta, a fellow Undine apprentice from the Himeya Company. Alongside her new friends, Akari strives to become a full-fledged Undine while taking in all the magical sights and sounds that Neo-Venezia generously offers.