Conan and Heiji investigate how only one piece of an evenly cut cake was poisoned. They discover a biodegradable capsule in a tea cup, suggesting the poison was delivered through a clever concealment method.
The episode narrows the focus to five people close to the family—Serika Wakamatsu, Masashige Shiina, Sumio Fujinami, Yoshimi Satake, and Sakurako Yonehara—all of whom have an "S" in their name, matching a clue in the dying message. Key Plot Developments
This arc is adapted from Files 781–786 in Volume 74 of the manga series.
This arc is notable for the early appearances of Masumi Sera , who continues to observe Conan’s detective skills. The Design of Poison and Mirage - Detective Conan Wiki
The investigation takes a darker turn when Serika Wakamatsu is herself found dead in her office from cyanide poisoning at the end of the episode.