Coming out of a restaurant, Mouri, together with Ran and Conan, meets his friend Dacun. Dacun says there has been a strange noise out their apartment these days. Mouri decides to go and investigate.

Since meeting an old friend, Mouri just keeps drinking. Ran and Conan go out to buy beer. On the way back, they see a man smoking in the darkness for the street lamp near him is already broken. Conan also notices only the light in Dacun's room is on.

Back to the room, they suddenly hear a noice of broken glass. Then the man whom Ran and Conan saw is found died. In Dacun's neighbouring room, some college students are taking a party, and the dead man is their classmate. One of them is a mountain climber. Since he was apart from his company, and write thesis in the internal room, he is suspected.

But he mustn't have left that room for the window in that room could not be opened wide. Conan thinks carefully. He finds the broken glass and the parapet of the opposite building roof also have blood sign. Now Conan knows how the mountain climber managed to murder.

First, the murder tied two fishing thread on a scree and shoot it to the roof of the opposite building with a slingshot. And he goes to that building, unloose the thread, and make one of them go from beyond the parapet. Then tied the end of the two thread. At the other side of the thread, he tied a rope on one of them and pull the other, thus the rope goes between the two buildings. Next he tied a pocket of scree on one end of the rope and threw it at his classmate. Then by pulling the other end of the rope, be brought back the lethal weapon. The sound which Dacun often heard and the broken glass are made by the pocket, when pulling back, hit the building. The mountain climber broke the street lamp and turned off the light in his room though he said he was writing theses, lest others should see the progress of the murder. He preengaged his classmate to wait there, and he is also able to aim the target in darkness because of the light caused by the smoke. The sock the climber wears is substantial enough to hold the screes and after murdering, he threw the screes into the goldfish pool. The evidence is the fibre in the pool which has the same color with is sock.