本編の1年前を舞台とし、早乙女メアリーを中心とした物語。Set one year before the main story and centred around Saotome Mary. Saotome, a bright young girl born into an ordinary family, has passed her transfer examinations and started to attend Hyakkaou Private Academy. Its an elite school with a twist: gambling has become a central part of its academic activities. Saotome adapts quickly to this outlandish system, and will soon succumb to the gambling mania that she comes to be known for in Kakegurui.