S1·E655 The Design of Poison and Illusion (Illusion)

Conan and Heiji's investigation leads them to realize how Ikuro and Serika were murdered. They reveal that when two pieces of the baumkuchen were left, they were rearranged using the Jastrow illusion and applied poison to the seemingly larger piece. For Serika's murder, the same culprit used the Gestaltzerfall phenomenon while she was signing contracts. This caused her to forget how to write the Waka (若) kanji forcing her to open a dictionary to the Wa (わ) pages where poison was applied. They reveal the culprit to be Satake, evidenced by the shine on her bronze watch due to a chemical reaction with cyanide. Satake confesses to the murders and explains it was to avenge Wakamatsu who was secretly her father. In the aftermath, Kazuha attempts to confess her feelings to Heiji but fails when he leaves partway.