The 26-year-old Kano Aki is found dead in her apartment after drinking coffee poisoned with cyanide. The police initially assume suicide since she left no farewell note, but an investigation report from the Mori Detective Agency lies on the table. This reveals that her boyfriend Sakon Yusuke entered a new relationship with the wealthy Kuga Michiru. Aki had apparently hired the detective Kogoro to check Sakon's loyalty.

Conan doubts the suicide theory, especially after Aki’s search history shows terms like "stalker" and "negligent homicide." Research reveals that Aki stole the cyanide from her workplace, Sotoba Chemicals. A conversation with her friend Nishida Hiroko reveals that Aki said the night before: "I will travel with the one I love"—a hint at a planned double suicide.

Conan and Kogoro eventually expose Sakon’s devilish plan: he manipulated Aki by disguising himself as a stalking stranger and frightening her. He then staged an "accident" in which the supposed stalker died and persuaded Aki to commit a double suicide to get rid of her and marry Michiru. Sakon secretly poisoned only Aki’s coffee, deleted her farewell letter, and instead placed the investigation report as the supposed motive.

Through Conan’s observation of a suspiciously placed cup and the analysis of surveillance footage, Kogoro reveals that Sakon himself made the call to the detective agency—under a fake name—to fuel Aki’s distrust. Sakon confesses that he saw Aki’s obsessive love as an obstacle to his future plans. The episode ends with his arrest, while the true circumstances of Aki’s tragic death are clarified.