High school detective Shinichi Kudo visits the Beika Aquarium with his childhood friend Ran Mouri. Their outing has a dual purpose: it is a date, but they are also using the time to devise the Aquarium Surprise Reunion Love Revival Operation, a plan intended to help reconcile Ran's estranged parents, Kogoro Mouri and Eri Kisaki. The lighthearted atmosphere shatters when Shinichi senses an unusual tension in the crowd. Guided by the faint smell of blood, he moves through the aquarium and discovers a man who has been fatally stabbed in the heart.

Shinichi immediately instructs Ran to alert the staff and secure the scene. Inspector Megure and his officers arrive to find the young detective already in command of the investigation, having used the victim's own phone to quickly identify him as Kunihiro Shumoto. By examining the contents of a receipt and the scent of perfume on the victim's shoulder, Shinichi deduces that Shumoto was with his girlfriend. He narrows the field of suspects to three individuals. Despite each suspect presenting video alibis, Shinichi exposes Naho Ojiro as the killer, demonstrating that the difficulty of filming long fish like sharks made her alibi video implausible. The crucial evidence is the blood on her glove, which she attempted to conceal by turning it inside out, inadvertently changing its color.

After the case is solved, Shinichi and Ran walk away from the aquarium. As it begins to rain, Ran puts up her hood, and her phone falls out of it, disappearing down a drain. Shinichi admits he placed the phone there earlier to test a theory about the murder. To make up for the loss, he promises to take her to Tropical Land. Still upset, Ran agrees on the condition that Shinichi pays for everything and that she wins her next karate tournament. The story then shifts to show Shinichi scouting Tropical Land in advance to plan the perfect day. As night falls, he pauses at a fountain and observes Kaito Kuroba and Aoko Nakamori enjoying ice cream together, a moment that marks a fateful intersection of their lives before the events that will transform him into Conan Edogawa. This special episode serves as a prequel, revisiting the final days of Shinichi Kudo's original life and leading directly into the series' iconic first episode.