In a mystical realm beneath the human world, a young girl named Chun transforms into a red dolphin to explore the human world. She rescues a human boy from drowning but sacrifices her own life to save him. Chun bargains with the Soul Keeper to return the boy to life and takes on a dangerous journey to nurture the boy's soul, which has manifested as a baby dolphin. Along the way, she must confront the consequences of her actions and make sacrifices to save her own world from destruction. With the help of her friend Qiu, Chun embarks on a perilous quest to return the boy's soul to the human world and find a way back home. But as she faces challenges and danger, Chun learns that the cost of love and sacrifice can be higher than she ever imagined. Will she be able to find a way to save both her world and the boy she has grown to love?

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Some movies that have similar genres and were produced around the same time as Big Fish & Begonia include Raya and the Last Dragon, Away, Wolfwalkers, Checkered Ninja, Epic, KonoSuba: God's Blessing on this Wonderful World! Legend of Crimson.