Kamishiro Kanna was summoned to another world to be a hero; but the one who summoned him was a scheming princess. When it was revealed that his mana measurement showed 0, he was put in prison. Together with a half-elf big-breasted silver-haired bishoujo he met in the palace's prison, they escaped riding a dragon that she summoned. The thing Kanna didn't know is that 3 days after his escape from the palace prison, the princess did another summoning and summoned 3 high school students, which happen to be his close friends.