Sonoko's older sister, Suzuki Ayako, is getting married to a man named Tomizawa Yuuzou. The Suzuki family and the Tomizawa family have always been good friends and have neighbouring summer houses. Yuuzou's father, Tetsuharu, is visiting his summer house so he can meet Yuuzou's new fiance. Sonoko has invited Ran and Conan along too. After a meal in the evening at the Suzuki house, Tetsuharu is killed whilst returning to the Tomizawa house. Tetsuharu's sons are identical triplets and one of them is spotted committing the murder. How do they establish which one is guilty?