Detective Mori Kogoro, his daughter Ran, and Conan are summoned by the lawyer Oi Hiroki to the harbor town Urasawa in northern Kyoto. The local ryokan manager Tsuji Kanta asks them to solve the mysterious death of his friend Yamaoka Takeshi, who was found dead two weeks ago.

Takeshi was found floating in a boat, his hair was dyed snow-white and he was wearing a turtle shell armor. Next to him lay a jewel-encrusted box, which immediately led the locals to attribute it to the curse of Urashima Taro, a local legend.

Kogoro begins the investigation and questions four suspects, all of whom had a motive:

- Matsuyama Hironobu, a fisherman who frequently had professional disputes with Takeshi about the future of fishing in the town.
- Nishino Miku, the operator of a luxurious ryokan. She was the victim's sister-in-law, as she was married to Takeshi's brother Kazuo, who disappeared at sea four years ago. Takeshi suspected her of being responsible for his brother's death and had publicly announced that she wanted to kill him.
- Togashi Hitomi, Takeshi's ex-girlfriend, who runs a cafe and shows an unusual interest in the investigation.
- Kamikawa Akihide, the priest of the Urasawa Shrine. The jewel-encrusted box and the turtle shell were stolen from his shrine. He is convinced that the curse of Urashima Taro is real, but also harbors a strong suspicion against Nishino Miku.

During the investigation, the division of the town between traditional fishing and modern tourism becomes apparent. The investigators are followed by Togashi Hitomi, who claims to have found crucial evidence. Before she can reveal it, contact is lost.

Her body is found shortly afterwards, similar to Takeshi's, floating in a boat. Her hair was also dyed white, and she too is wearing a turtle shell. The case expands and ends with a cliffhanger that postpones the resolution of the murder of Takeshi and now also of Hitomi to the next episode.