Tanshin Hanabi follows Sakuragi Shun, a salesman enjoying a happy life with his wife and daughter. His world changes abruptly when a sudden personnel shift transfers him to Kagoshima, a city he remembers well from his junior high school days.
After seventeen years, Shun unexpectedly reunites with his first love, Takeda Hana. She now works as a junior high teacher and lives mostly alone, as her husband spends much of the year at sea. As they reminisce about their bittersweet high school memories, a hidden Pandora's box of emotions begins to open. Further complicating things, sparks also fly with Katoyama Naoya, Shun's former friend and love rival from junior high, who now works in the same industry. Despite having a beloved wife, Shun finds himself quietly grappling with jealousy.
Meanwhile, Shun's wife, plagued by anxiety and suspicion from their phone calls and emails, becomes convinced he is cheating. To eliminate her fears, she takes a trip to visit him in Kagoshima. As small lies begin to accumulate, the truth becomes increasingly complicated. When these lies are eventually exposed, the film explores how the relationships between the four individuals will irrevocably change.
Other names: 単身花日, たんしんはなび, Single Flower Day