Owing to an increasing debt, Kaiji Itou ends up resuming his old lifestyle. One day, while walking on the street, he stumbles upon Yuuji Endou, who is hunting Kaiji due to the money he owes to the Teiai Group. Unaware of this, Kaiji eagerly follows Endou, hoping for a chance to participate in another gamble, but soon finds out the loan shark's real intentions when he is kidnapped. Given that Kaiji is unable to pay off his huge debt, the Teiai Group instead sends him to work in an underground labor camp. He is told that he will have to live in this hell for 15 years, alongside other debtors, until he can earn his freedom. His only hope to put an early end to this nightmare is by saving enough money to be able to go back to the surface for a single day. Once he is there, he plans to obtain the remaining money needed to settle his account by making a high-stakes wager. However, as many temptations threaten his scarce income, Kaiji may have to resort to gambling sooner than he had expected. [Written by MAL Rewrite]

Unlike many schools, attending Hyakkaou Private Academy prepares students for their time in the real world. Since many of the students are the children of the richest people in the world, the academy has its quirks that separate it from all the others. By day, it is a normal school, educating its pupils in history, languages, and the like. But at night, it turns into a gambling den, educating them in the art of dealing with money and manipulating people. Money is power; those who come out on top in the games stand at the top of the school. Yumeko Jabami, a seemingly naive and beautiful transfer student, is ready to try her hand at Hyakkaou's special curriculum. Unlike the rest, she doesn't play to win, but for the thrill of the gamble, and her borderline insane way of gambling might just bring too many new cards to the table. [Written by MAL Rewrite]

As Yumeko Jabami's fame grows and the reputation of the student council dwindles, Kirari Momobami decides to revolutionize the group. To this end, she announces an election for its next president. The rules are simple: each student in the school receives one chip. Whoever has the most chips by the end of thirty days becomes both the new president and the head of the Momobami clan. Upon receiving news of this development, the Momobami branch families spring into action. Eleven transfer students arrive at Hyakkao Private Academy, each aiming to lead both the school and the Momobami clan. Equipped with unique talents, they will compete to get as many chips as possible—but their chips are not the only things on the line. [Written by MAL Rewrite]

Tokiko Mima, nicknamed "Key," is a 17-year-old girl living in the Japanese countryside who, despite her human-like appearance, is a robot. When Key's grandfather Dr. Murao Mima passes away, he leaves her a dying message, telling her that she can become a real girl if she is able to make thirty thousand friends. Thus, Key moves from the quiet Mamio Valley to the busy streets of Tokyo, where she soon runs into her childhood friend Sakura Kuriyagawa. Key quickly becomes enamored with idol singer Miho Utsuse and wonders if becoming a singer will allow her to make the amount of friends needed for her to become human. But Miho carries a ominous secret: she is connected to Jinsaku Ajou, an old rival of Dr. Mima trying to make new a breakthrough in robotic weaponry. As Key works to become a real girl, Ajou sets a dangerous plan into action, and it turns out there's much more to Key than meets the eye. [Written by MAL Rewrite]