The Design of Poison and Mirage
 Characters introduced
Heiji reads a letter he received from an already dead man and old acquaintance of his mother, design company president Kouhei Wakamatsu, who was killed a month earlier in his Karuizawa villa.
Heiji proves the letter was sent by someone else by showing the postmark was mailed ten days after the murder, which occured during the wife's birthday party in the restroom. Heiji reads the letter from the culprit wanting Heiji to solve the crime, and is given the house key to the villa in the letter. Kazuha went with three of her friends to look at the home in Osaka while Heiji had a cold. They find a strange looking man in the bathroom who vanished when the lights went out for a moment. Kazuha remembers seeing the word "EYE" scratched in the wall written in english. Heiji went to take a closer look after feeling better, but the house was being rebuilt. He points out that Wakamatsu's dying message was erased when police arrived as evident by the blood on the victim's finger but no smear. Ran notes that the deceased could have had glue, which was true since he was re-tiling his bathroom at the time, identical tiles to the house in Osaka. The culprit could have flipped the tiles to hide the message. The tiles had a coloring effect where they would get lighter from top to bottom. Everybody, except Sera who wasn't invited, upon reaching the home of the dead man's widow, Serika Wakamatsu, and the son of the dead president, Ikurou, bursts in demanding dinner. Two more men arrive: Masashige Shiina, the senior managing director, and Sumio Fujinami, the chief designer. Masashige brought baumkuchen, a kind of cake, and Fujinami blurts how he tasted a piece to assure everyone it isn't poisoned. Serika calls for the secretary, Yoshimi Satake, to make some tea while everyone waits. In the living room, the housekeeper Sakurako Yonehara says she saw the dying message before it was erased. His blood was used to write the letter "S". During the cake cutting, Ikurou cuts to the chase wanting to discuss the new company president. As he is explaing why he should be president, he picks up a piece of baumkuchen and drops dead from poison in front of everyone.
The police arrive and investigate how the cake piece that Ikurou ate was the only poisoned piece. Conan points out a capsule in a tea cup which could have been what carried the poison. As the pill is biodegradable, the empty capsule was put in the tea after the poisoning so destroy the evidence. The baumkuchen was cut evenly into eight pieces, the only poisoned piece was that consumed by the victim, and Heiji takes notice of the "S" in the dying message. Narrowing down five suspects: Serika, Sumio, Shiina, Satake, and Sakurako.
As the police interrogates the suspects while the widow, Serika Wakamatsu, is excluded since she was never close to the baumkuchen tray. Meanwhile, the police detectives in Nagano reveal to Heiji that Wakamatsu's killer reorganized the bathroom tiles to hide the dying message. The tiles used the crazy diamonds optical illusion giving the impression that it could not be rearranged without breaking the pattern. When arranged back to normal, the message reads "SON" indicating Ikurou was Wakamatsu's killer.
Conan and Heiji resume their investigation while Ran and Kazuha gossip about Shinichi's confession towards Ran. Conan and Heiji discover powdered poison on Ikurou's doorknob and Serika's slippers; the evidence indicates her as Ikurou's murderer. The two confront Serika whom is found dead in her office from cyanide poisoning.
Wakamatsu's secretary, Yoshimi Satake, reveals that per Serika's will, a letter of confession to Wakamatsu's murder is to be given to the police upon her death; the letter confirms Ikurou was the murderer and Serika's role in hiding the dying message.
Conan and Heiji's investigation leads them to realize how Ikurou and Serika were murdered.