The Junior Detective Club were at the beach digging up clams. They encountered a college group of students on similar excursion. Later, one of the college boy was found dead from poisoning. Was it suicide, or murder?

Mori Kogoro solves the case! It is a busy day for the detective when he plans to meet a client and watch the new Okino Yoko drama series. His assignment is to decipher a suspicious text message written in code by the client's daughter. However, Mori finds out there is a cat in the office. It is Goro, a Russian Blue that belongs to Eri. While Eri has to Okinawa for a case and Ran and Conan go to school, Mori has to look after the cat. Mori becomes annoyed with Goro when gets cathair over the wash towel, knocks down an English-to-Japanese dictionary, plays with shoes, and goes into the video collection. But through these hints, Mori is able to figure our the coded message! It turns out Conan hid katsuobushi in several places so Goro could give hints to Mouri. In the end, Eri shows up to pick up Goro.