"Kakegurui Twin" is a prequel to the original "Kakegurui" series, set one year prior to the events of the main storyline. The narrative centers on Mary Saotome, a second-year transfer student at Hyakkaou Private Academy, an elite institution where students engage in high-stakes gambling to establish social hierarchy. Unlike the original series, which focuses on Yumeko Jabami, "Kakegurui Twin" explores Mary's rise within the school's cutthroat gambling culture.

Mary arrives at Hyakkaou with a reputation for being a skilled gambler, but she quickly realizes that the academy's gambling system is far more complex and ruthless than she anticipated. The school operates on a strict hierarchy where winners gain power and influence, while losers are subjected to demeaning "pet" status. Determined to climb the ranks and avoid humiliation, Mary uses her sharp intellect, strategic thinking, and willingness to take risks to outmaneuver her opponents.

The series delves into Mary's early interactions with key characters who later play significant roles in the main series. These include Tsuzura Hanatemari, a kind-hearted but naive classmate who becomes Mary's first friend and ally, and Mikura Sado, a submissive student who serves as a pet to the powerful student council member, Itsuki Sumeragi. Mary's rivalry with Itsuki becomes a central conflict, as Itsuki views Mary as a threat to her own position and attempts to undermine her at every turn.

Throughout the series, Mary faces a variety of gambling challenges, ranging from card games to psychological battles, each designed to test her resolve and adaptability. Her journey is marked by both victories and setbacks, showcasing her growth as a gambler and her increasing understanding of the academy's brutal social dynamics. The narrative also hints at the origins of Mary's competitive and calculating personality, providing context for her actions in the original series.

"Kakegurui Twin" maintains the tension and psychological intrigue of its predecessor while offering a fresh perspective on the world of Hyakkaou Academy. By focusing on Mary's early experiences, the series enriches the overarching narrative of "Kakegurui" and provides deeper insight into the motivations and relationships of its characters.