A threatening letter signed "the Guardian of Time" warns that Rukako Hoshina, the mistress of a large mansion with a clock tower, will be murdered during her birthday party at 6:00PM. Kogoro and Conan goes to protect her but fails. It appears that the culprit stabbed her with an ice pick and fled from the balcony, but shoe prints and the murder weapon are never found. According to witness testimonies, the culprit is in a dress who moved nimbly, yet is plump and of a large build, yet no one in the party matches the description.