Manabu Yukawa is invited for a presentation on Submarine Mineral Resources Development in Harigaura. He stays at an inn. Manabu meets Kyohei who came to the inn, which is run by his aunt's family, for summer vacation. The next morning, Tsukahara, who is another guest at the inn, is found dead on the bank of a port. Manabu becomes involved in the case.
