Riri to Kaeru to (Otouto)

A girl named Riri visits an amusement park featuring a frog mascot with her parents. Inside the park's Frog Castle, she finds and draws a sword. According to legend, the Frog King will grant a wish to anyone who defeats a large snake with the sword. After Riri defeats the snake, she enters the Frog Land and meets the Frog King, who tells her she will turn into a frog if she fails to open the Gate of Hope before a child hatches from the roe in front of her. A man in a suit named Knodo appears and is assigned to be her little brother. Riri finds this odd and tries to escape, but he follows her, and both end up falling into the water.