In this episode, the Detective Boys investigate an alleged ghost phenomenon occurring on the roof of the TOMO restaurant. The student Mamiya Mami has been observing a ghostly apparition with will-o'-the-wisps above the roof every Thursday for two weeks and asks the Detective Boys for help.

At the same time, the live-alone Motoko Kanazawa, whom Mami assists, has been receiving silent calls since the same time. The children discover large, round footprints around Motoko's house that do not belong to her. During their investigation, they find out that the restaurant roof is actually not accessible because the door is broken.

Conan and Haibara join them and realize that the "ghost" must be a human who reaches the fire escape to the roof via stacked delivery boxes. The perpetrator uses the weekly deliveries on Thursday to get onto the roof undetected. The ghostly apparition is created by a person with black outer clothing and a white apron, using a telephone as a "will-o'-the-wisp".

It turns out that the head chef of the TOMO restaurant, Tomoharu Kanazawa, is the son of Motoko. After an argument years ago, he had cut off contact and was secretly observing his mother from the roof. The silent calls came from him to check if she was at home. The footprints were made when he hesitated to enter the house. Motoko's statement in a newspaper article that she had no children was only a ruse to catch con artists and had mistakenly hurt Tomoharu.

The episode ends with mother and son reconciling and the case being solved.