Seiko Nagare, a 17-year-old high school student suspended from her Tokyo private school for defending a friend against bullies, embarks on a solo trip to Nagasaki accompanied by her black-and-white cat, Gonbei. During the train journey, she encounters a young guitarist named Takano Kyouichirou, only to later read a newspaper reporting his murder days prior, casting doubt on his true nature. She also meets Chuuta Misora, who develops persistent romantic interest in her, advances she consistently rejects throughout subsequent events.

Upon arriving in Nagasaki, Seiko independently tours landmarks including Urakami Cathedral, the Peace Park, and the Dutch Slope. Her explorations lead to the discovery of a hanged woman in nearby woods, confirming murder over suicide. This draws her into an investigation connected to the Totsugawa family and individuals from her train journey. Despite initial hesitation, she accepts Chuuta's assistance in unraveling the mystery.

Her cat Gonbei remains a constant companion, occasionally providing comic relief through uncharacteristic behavior like riding obediently in her backpack and performing human-like gestures. The murder case resolves without full closure on secondary elements such as Chuuta's background or the significance of playing cards referenced in the title. Her appearance and development in other media remain undocumented.