Born into an ordinary family, Mary Saotome (Aoi Morikawa) is thrilled on her first day transferring to Hyakkaou Private Academy. This is no ordinary school—it's a prestigious institution attended by the children of political and business elites, where Mary's dream of living a refined, elegant life is about to begin. Welcomed warmly by her new classmates in Hana Class 1, Mary feels optimistic. However, the class's central figure, Kokoro Aiura (Riko Fukumoto), invites her to participate in something unexpected: gambling. At this academy, academic or athletic achievements mean nothing—gambling prowess reigns supreme. Mary's first taste of gambling in a game called "Speed President" ends in a crushing defeat. Though she initially tries to escape, an encounter with her former classmate, Tsuzura Hanatemari (Shiori Akita), sparks a new resolve. Thus begins Mary Saotome's journey down the path of gambling.

Returning to the gambling den with ¥300,000 borrowed from Tsuzura, who was treated like livestock in class, Mary's eyes gleamed with a completely different intensity than before. "With this money, let's have another round of gambling!" she declared, challenging Kokoro to a rematch. The gambling battle began anew. Kokoro, setting a trap and holding an absolute advantage, seemed untouchable. Yet, despite the overwhelmingly unfavorable situation, Mary kept raising the stakes and pressing on, embodying the quintessential "self-destructive" gambler. However, she had a cunning strategy up her sleeve, and the tide of the match began to shift in an unexpected direction... This was none other than the unmistakable moment of awakening for Mary Saotome, the high school girl gambler who would become a central figure in the series of incidents that would shake Hyakkaou Academy a year later.

Determined to become a winner at Hyakkaou Academy and free Tsuzura from her "housepet" status, Mary devised a plan to open her own gambling den and become its master. To do so, she targeted the least popular gambling den in the school, run by the Literary Club under the leadership of Yukimi Togakushi (Minori Hagiwara). Mary and her group stormed the club, only to encounter unexpected visitors: the Student Council Disciplinary Committee Chair, Sachiko Juraku (Hajime Nagai), and her "housepet," Mikura Sado (Mirei Sasaki). They had come to seize the Literary Club's gambling den due to unpaid usage fees. In the midst of this, Mary made a shocking proposal to Sachiko, offering to take over the gambling den herself. Intrigued by her boldness, Sachiko agreed, but set a condition: Mary and Yukimi would face off in a gamble, with the loser taking on the Literary Club's debts. Thus, Mary's second battle began.

Sakura Miharutaki (Erika Ikuta), the Student Council Beautification Committee Chair and master gambler, faced off against the gambling den-wrecking freshman, Midari Ikishima (Miki Yanagi). The two competed in a game of "Chō-Han Bakuchi" with the gambling den's usage fees at stake. Who would emerge victorious: the "Iron Lady" or the "fear-inspiring freshman?" Meanwhile, having formed an odd friendship (?) after their previous confrontation, Mary, Tsuzura, and Yukimi devised a plan to revive the Literary Club's gambling den. Their strategy? The trio, dressed as maids, would act as dealers in a dice-based gambling game called the "Magic Dice Game." The setup became a massive hit among male students, and the den quickly flourished. However, just as things were thriving, another Student Council member appeared…

To avoid the forced closure of the Literary Club's gambling den, the student council treasurer, Aoi Mibuomi (Sano Hayato), orders Mary to face off against a designated opponent. That opponent is Nagi Kamishimo (Atsuhiro Inukai), a top-ranked, formidable gambler at the academy known for reading people and leaving them exposed. Nagi displays an arrogant attitude towards Mary as she takes on the challenge. The game they play is "Full Count Blackjack," a variant of Blackjack using 13 cards, demanding advanced psychological tactics. "I'll break your spirit," Nagi declares, provoking Mary from the start. Losing her composure and falling behind, Mary faces an uphill battle, but the outcome of the match takes an unexpected turn.

After her intense battle with Nagi, Mary is taken by Aoi to a secret place. There, she finds many students, including housepets, as well as Sakura and Nagi. Aoi declares that this organization, called "Full-Bloom," aims to overthrow the student council, which controls the academy through gambling, and its president, Kirari Momobami (Elaiza Ikeda). Aoi, who is gathering money from skilled gamblers to topple the student council, invites Mary to join Full-Bloom. Hesitant, Mary asks for a day to consider her response. Meanwhile, various schemes and motives intertwine within Full-Bloom, and the situation veers in an unexpected direction.