Conan, Genta, Mitsuhiko, Ayumi, and Ai go to the site of a demolished mansion owned by Professor Agasa's acquaintance to practice soccer, but when they get there they find the police conducting an investigation. The body of a man was found floating in the pool inside the lot. He died to a blow to the back of the head by a blunt object, and the gate to the lot was unlocked by Agasa, who arrived earlier. The lot is surrounded by a brick wall, and on top of the wall are glass shards and barbed wire. The gate was last opened three days prior, but the victim is presumed to have died before midnight last night. The incident occurred in a roofless locked lot. The Detective Boys try to tackle the locked room mystery, but...