Mary walks into Hyakkaou expecting a coronation. She gets a basement instead. The show builds its tension not through elaborate mind games but through watching someone with nothing figure out how to survive people who've been groomed to destroy. By episode two, you're not watching a gamble, you're watching a girl earn every inch of ground she takes, and that's worth way more than the psychological warfare porn of the main series.

Mary Saotome enters Hyakkaou Private Academy expecting a guaranteed path to the top. At this school, students prove their worth by gambling, and those with the smallest winnings sink to the bottom of a harsh hierarchy. Meeting an old classmate who has been reduced to a housepet draws Mary into that world. Bold, defiant and sharp-witted, Mary scores a decisive win against a classmate that catches the eye of the scheming student council. Her childhood friend Tsuzura Hanatemari faces a bleak future near the bottom of the rankings, so Mary sets out to climb them and earn enough money to free Tsuzura from the system, beating even strong opponents along the way. Some council members seem to have larger plans for her. A psychological drama with mystery elements, the ONA is a prequel to Kakegurui.