Lily to Kaeru to (Otouto)

Lily went to an amusement park using the frog as its symbol character with her parents. At the Frog Castle of the park, she happened to draw a sword. It was said that the Frog King would grant any wish if one beat the big snake with the sword. When Lily beat the snake, she went to the Frog Land and met the Frog King. The king said she would become a frog if she couldn't open the Gate of Hope until a child would hatch from the roe in front of her. Suddenly, a man in suits appeared. His name was Knodo, and he was supposed to become her little brother. Lily thought it strange, and she ran away from him. He chased her. Then, they fell over into the water head over heels.

(Source: AnimeNfo)