St. Dominic's Academy is a high-class boarding school for the children of the global elite, where traditional education has been replaced by an underground gambling system that dictates the entire social hierarchy. Students wager vast sums of money, and those who accumulate debt become "house pets," forced into servitude to the students who defeated them. The school is ruled by a terrifyingly powerful Student Council, whose members hold the top ten spots on the leaderboard and enforce the school's ruthless code.

Into this world walks Yumeko, a mysterious transfer student whose cheerful and compulsive gambling addiction masks a dark secret. Yumeko arrives at St. Dominic's not merely for the thrill of the wager, but on a quest for revenge. Years ago, she witnessed her parents die in a car bomb explosion, and she has discovered that those responsible have connections to the school's board of directors. Her plan is to climb the gambling ranks, join the Student Council, and ultimately gain access to a secretive board event where she can confront the people who murdered her family.

Yumeko immediately disrupts the status quo by defeating Mary, a top-ranking council member, in a card game called Skirmish. Using her wits to overcome Mary's cheating, Yumeko wins a massive bet and frees her new ally Ryan from being Mary's pet. Ryan, an infatuated and kind-hearted student, becomes Yumeko's closest companion, helping her navigate the complex and dangerous rules of St. Dominic's. Mary, stripped of her council status, eventually forms a reluctant alliance with Yumeko, finding a new respect for her fearless approach to gambling.

Throughout the first season, Yumeko faces off against the other eccentric and sadistic members of the Student Council. She plays Ice Breaker against the influencer Suki, a game where heart rate monitors expose hidden truths. She challenges the violent Dori to a game of First Blood, a duel with weapons where drawing first blood determines the winner. The stakes escalate dramatically during school-wide events like House Wars, a capture-the-flag style competition, and the House Pet Hunt, a brutal "Hunger Games" style event where pets and owners are handcuffed together and forced to hunt for valuable eggs.

The central antagonist is Kira, the cold and calculating Student Council president, whose father sits on the school's governing board. Kira sees Yumeko as a threat to her power and orchestrates several attempts to destroy her, including forcing Yumeko into pet status. In the season's climax, Yumeko and Mary are poisoned by Kira and forced to play a high-stakes game of Blackjack, with the antidote as the prize. In a desperate final gamble, they force a draw with Kira, securing the antidote and surviving. The season ends with Yumeko's revenge unfulfilled, her parents' killers still unidentified, and the power structure of St. Dominic's fundamentally shaken but not yet broken, setting the stage for a second season.