On the eve of the Touto Line opening, the Governor of Tokyo, Yuuichiro Asakura, receives a threatening letter that casts a shadow over the ceremony. At the ceremony, Conan Edogawa, guided by the Detective Boys and accompanied by Hiroshi Agasa, rides beneath the gleaming tracks of the new line. Aware of the danger, Conan uses his original voice to alert the police as Shinichi Kudo, stopping the train and redirecting traffic away from the explosives, preventing any deaths.
With the immediate threat contained, Conan digs into Asakura’s political history and discovers the governor oversaw the dam construction in Kitanosawa, a town altered forever when the dam drowned the old village and reshaped the lives of its residents. The bombing adds a new layer of urgency, since Conan suspects the bomber’s trail could be tied to Kitanosawa, prompting a plan to travel there and seek the truth.
The investigation leads Conan, the Detective Boys, Ran Mouri, Kogoro Mori, and Sonoko Suzuki to Kitanosawa, where they are welcomed by a local circle: Keisuke Yamao, Mizuki Toono, Fuyumi Tachihara, Takehiko Muto, and Shogo Hikawa. The five reveal that they were childhood friends who grew up together in the shadow of the town’s relocation after the dam’s construction. Eight years earlier, Yamao was jailed for killing Toono’s sister, Natsuki Toono, in a hit-and-run, and the gathering marks a fragile return to a past that will soon entangle them all again.
The next day, Touma Tachihara awakens from an eight-year coma with no memory of the accident that haunted him, heightening the tension as old resentments and resentful secrets begin to surface. Shogo Hikawa is found dead in a field, and the initial assumption of suicide gives way to a clearer picture: Hikawa was killed by an electroshock device, and the killer waited for fresh snow to cover the scene, leaving backwards footprints to mislead investigators. A newspaper clipping about a jewelry-store robbery surfaces during Conan’s search, and a destroyed cell site further narrows the field of suspects. Meanwhile, the Detective Boys and Touma become targets of a sniper, forcing Conan to intervene and guide them to safety.
As the clues converge, Conan pieces together that Yamao is the culprit behind the bombings and the broader plan to retrieve the diamonds. A grim reveal follows: Touma recalls seeing Natsuki Toono pushed into the road by someone, and Toono herself confesses that she was the sniper, living with the brutal truth that the death of Natsuki was not a simple accident. Yamao, already driven by a desperate motive to find the diamonds, agrees to turn himself in after burying the jewels on a property in Kitanosawa.
The conspiracy escalates as the dam becomes the focal point of the final confrontation. Yamao’s bomb threat forces a race against time, and Conan engineers a dramatic diversion by carving grooves in a steep mountainside with his motorized snowboard to trigger an avalanche that would redirect the dam’s flood away from Kitanosawa. The plan succeeds in diverting most of the flow, but a short-circuit traps Conan in the midst of the rising snow. The rescue comes when Ran’s call on Shinichi’s cellphone pierces the silence, reviving Conan’s consciousness and allowing him to relay the critical warning to his friends. With the avalanche arcing to safety and the danger momentarily quelled, the group faces the aftermath: trust is rebuilt, memories are faced, and the long shadow of the dam’s legacy begins to recede as the investigation moves toward closure.
