Long ago, there lived a Princess. Her name was Princess Yuuka. She was the youngest of her three siblings in the East Kingdom of Shizukesa (Kingdom of Tranquility). Princess Yuuka was born with a strange condition. A curse, the Kingdom wise-woman deemed it. Yuuka was a sensitive soul. So sensitive that if she felt a pain or feeling she was unfamiliar with, her heart would shatter. And she would die. Thus she was known as the Glass Heart Princess. And for this reason, King Akio III, and his wife, Queen Hiromi kept Yuuka isolated from the world her entire life. But one day, Princess Yuuka escaped from the castle because she heard how beautiful the village below was from one of the guards, and she met a prince. They instantly fall in love with each other. But one day the prince is told that he is not worthy enough to love the princess, so he distances himself from her. This causes the princess to think he doesn't love her and her heart slowly cracks. If her heart was to shatter, she would, of course, die.

A 14 year old girl named Umiko Inihara, who lives on the surface along with all the other humans and her best friend, Satchiko Katanayagi, when one day some boys from Umiko's school bully her, and she runs to the sea port, where they corner her, and she falls into the sea. After 5 minutes of not being able to breath, she gets surrounded by magical fish called the Seikatsu. The Seikatsu trap fallen victims and look into their memory. If their memory is of any use to them, they give them the ability to breathe underwater, if not, they leave them to drown. The Seikatsu look into Umiko's memory, and find that she was born in the sea, and her mother was none other than Shinju-sama, Queen of Kiyokuni ōkoku, an underwater Kingdom buried deep in the sea for it's own protection. Which means Umiko is the princess. These memories are buried deep inside of Umiko's mind, which means she knows nothing about her mother or Kiyokuni ōkoku. All she rememers from the day she was born is opening her eyes to the bright lights of Minashigo Haven, the only orphanage in town, and being adopted by Hotaru Inihara. The Seikatsu give her back her memory, so when she gets back to the surface, she now has to discover a way to get to Kiyokuni ōkoku and find her mother in order to wake her and the rest of the kingdom from a deep sleep put over them by the evil sea demon, Akuma-Umi, who now resides on the surface, keeping a close watch over Umiko, trying to make sure that she never sets foot in Kiyokuni ōkoku and awakes her rightful kingdom. To help Umiko find her way, a knight, around the same age as Umiko, named Akio, is sent to the surface to help Umiko unravel the deadly and dangerous secrets hidden with Kiyokuni ōkoku. Eventually Umiko falls in love with Akio. Along the way, Umiko and Akio step into traps, and find themselves on the same path as Rin. A lonely school girl who might know a thing or two about Kiyokuni ōkoku, and how to find it.