The Detective Boys are initially busy finding a missing cat named Mu-chan for a client. During their search, they meet the detectives Takagi and Chiba, who are looking for a large man in work clothes. This man is used by Mr. Matoi as his alibi for a murder case in which his ex-girlfriend Jojima Shiori was killed.

Conan realizes that the "phantom giant" is Genta and Mitsuhiko, whom Matoi saw drunkenly leaning over a wall and mistakenly took for a large man. This, however, secures Matoi's alibi.

The investigation then focuses on the murder of Jojima Shiori. Conan concludes that the real culprit is her colleague Yumura Shohei. Yumura had embezzled company funds, and Jojima had blackmailed him. As a murder weapon, he used the plastic packaging of a wedding gift, with which he suffocated her. To direct the investigation towards Matoi, Yumura manipulated the crime scene circumstances.

In parallel, the Detective Boys are finally able to find the missing cat Mu-chan, thus successfully concluding both cases.