Akari Mizunashi, at the age of 15, has left everything behind to travel to Aqua, a sparkling planet covered in water. More than anything, Akari wants to be an "undine" - a female gondolier who navigates the canals of the Aquan city of Neo-Venezia. As she begins her training with the prestigious Aria Company, will she be up to the challenges that await her on the path to achieve her dream?

Two wonderful things have happened: it's time for Neo-Venezia's annual Carnevale, and Ai has come back to Aqua for a visit! As Akari and Ai wander through streets full of bright colors and beautiful masks, they catch a glimpse of Casanova, the mysterious master of ceremonies. Who is the man behind the mask? The two are curious, but they'll need to be careful. You know what they say about curiosity and the cat, after all...

While practicing one day, the girls see President Aria in his own little gondola. But he doesn't see them... instead, he disappears down a canal that's normally closed off. According to Alicia, there's an old legend on Manhome that tells of the Kingdom of Cats, a strange realm that people aren't allowed to enter. Could such a place really exist?

Many of the water touring companies in Neo-Venezia have their own palina - a tall, decorated pole used to hitch gondolas. But strangely, the Aria Company doesn't have one. When Akari asks Alicia about it, Alicia gives her full permission to make a palina of her own! It's tough work building something from scratch, but Akari is determined to make the best palina possible.

In the still of the night, Akari is woken up by the distant sound of a train. But Neo-Venezia's trains don't run at that hour, so what was it she heard? And to make matters even more mysterious, the next day, President Aria gifts her with a train ticket. It would seem Akari is about to have another brush with the secretive spirit, Cait Sith...