On Neptune the rain that falls is made of diamonds. It is said to be a love letter from Neptune's moon, Triton, to the shining blue icy planet. Upon hearing such a rumor, the legendary treasure hunter Château came to Neptune. He seeks a fierce storm and adventure, but when he makes landfall his destination is a beautiful dream world full of jewels and music, and he is unexpectedly welcomed. Triton, the King of the Moon, presents only two conditions to the bewildered Château. One is that he may touch anything except the diamonds. And the other is not to fall in love with Neptune, the Queen of Ice. So long as he follows these conditions, he can stay as long as he wishes. Château spends a peaceful time adventuring with the mysterious inhabitants to tunes that recall memories of his distant hometown in France. However, he is gradually fascinated by the beautiful appearance of Neptune, which disappears when it floats beyond the rain. Love turns rain into a storm.