The waitress of Poirot Cafe, Azusa Enomoto, reported she received strange messages from a boy, suggesting he might be kidnapped or trapped in a location without food and water. The only way to find him is by reading his text messages. The boy only knows how to send, but does not know how to receive. Conan must combine the clues he's given to find out his location.
The boy's first text mentioned a lot of people wearing hats and fish being caught in a net, implying that he is near a fishing spot.
Since the boy's following texts mentioned a restaurant sign, people wearing hats, and people peering down at him, he must be in a position which requires him to
look up. Also, because no one has yet to show concern for a boy left alone without his father for a long period of time, he must be inside a car.
The fifth text about there actually being three fish in the sky, not just one refers to Koinobori (carp streamers). Since one streamer became three, that implies that wind has picked up, and thus Mitsuru's location is Komaezaki.
The fourth text referring to fish being in a net is because a large net surrounding a practice golf course had overlapped with the carp streamers, thus seeming like a giant net in the sky with fish in it.
Conclusion
Conan and the others find Mitsuru passed out in the car. He comes to just before Ran kicks the door open, and agrees after persuasion to come out of the car. Kogoro overhears two men speaking about a fight that occurred earlier, with one ending up with an injury to the head. Shortly afterward, Mitsuru's father comes running. As it turns out, he had attempted to break up a fight, got knocked out and ended up in the hospital. Mitsuru's mother also arrives--as it turns out, she was hospitalized and had just been discharged. Mitsuru's father had come to Komaezaki together with Mitsuru to pick her up. Mitsuru was left without any knowledge of the true reason in order to keep it as a surprise and to prevent him from worrying about his mother.
At the end, Ran hints to Kogoro about how she wishes that she too wants to become (a family) of three again.
