A young girl is offered as a sacrifice to an evil old dragon to help in a battle against an evil villain. However, the dragon is a herbivore, not evil and actually cowardly.

A 5000-year-old vegan dragon was living peacefully when one day a young girl showed up in his cave. She offered herself as a sacrifice in order to gain favor for her village. He played along as the great "Evil Dragon, the Demon Lord's Army Leader" in order to get rid of her. However, his little white lie awoke her hidden powers and his peaceful life suddenly ended.—Bilibili TV

Desperate to defend her village from the demon king, Reiko offers herself to a 5,000-year-old dragon for his help in exchange, but the flame-breathing beast is vegetarian and a total weakling. Together they embark on a journey to take down the demon king. With her dragon comrade, Reiko claims that she's his disciple, able to draw from his infinite powers, but is it really from him?—Crunchyroll