The case in question is chapters 115-117 of Yankee-kun to Megane-chan.

The story starts off without a hint of mystery. The new student council is taking a trip to a beach-side inn to plan for the upcoming school festival alongside the old student council made up of the main characters. They blow through their meeting to get as much fun in the sun as they can and then relax at the inn when night falls. But when they find Izumi has been attacked and is covered in seaweed and fish. After talking to a local hooligan who works at the inn the group discovers they have triggered the wrath of a local sea-god. They barge into the only other occupied room in the inn and find an ominous message that they all shall feel the wrath of the curse. Soon they are being taken out one by one in an equally bizarre manner.  Can Shinagawa discover who or what is behind the attack before they are all subject to the brunt of this fearsome grudge?

At first everyone is a bit panicked. Certain members of the group assume it is a prank while others a quick to believe that something supernatural is afoot. Soon they are taken out whenever they are small groups. Chiba being the smartest of the group is the first to get an inkling of who the culprit is. He suspects it might be Shinagawa as Shinagawa is always the first to discover the victims but after judging his reaction by accusing him off to the side he determines it must be the work of someone outside the group. He then examines one of the suspicious room in the inn and finds damming evidence of the culprit. Sadly he makes the number one horror movie mistake and goes alone when he tries to find the evidence. But Shinagawa quickly determines the culprits by following Chiba’s train of logic and is able to unmask the distinctly mundane perpetrators before they can get him or the girls.

Overall this was not the most serious of mysteries but overall Yankee-kun to Megane-chan is not the most serious of manga series. But it is a 3 episode mystery that is fun while having a surprising amount of detective work for something so short. As you can see they even do the faceless criminal panel that they love to do so much in Detective Conan. It also has a bit of a Higurashi vibe because the only thing the Japanese love more than a good mystery is a good supernatural mystery. Also this gives me a chance to talk about Yankee-kun to Megane-chan again and it is just a series that need more love in general.

P.S. I know the last picture has nothing to do with the mystery but it is my article so I can add what I want!