In the near future, much of the world has been forgotten at the bottom of the ocean due to rising sea levels. Natsuki Ikaruga, who lives in a small town surrounded and stranded by the sea, is invited by a mysterious woman, Catherine, to search for a treasure in his grandmother's house who was a marine geologist. One day, he dives to the bottom of the ocean in a submarine and encounters a mysterious capsule with a girl sleeping inside. The girl was a humanoid so sophisticated that she could be mistaken for a human.
I am Natsuki Ikaruga's 
trusty partner, after all!
!أنا شريكة ناتسكي إيكاروغا الموثوقة في النّهاية
Ich bin Natsuki Ikarugas
Partnerin, weißt du?
Soy la socia de Natsuki Ikaruga.
Soy la socia de Natsuki Ikaruga.
Tu t’adresses quand même
à la partenaire de Natsuki Ikaruga.
Io e il qui presente Natsuki Ikaruga
siamo partner!
Eu sou a parceira fiel do
Natsuki Ikaruga, afinal!
Я, между прочим,
партнёр Нацки Икаруги!