Maupiti Island

Maupiti Island is set in the year 1954. The premise of the game begins with Jérôme Lange on a trip to Japan from Madagascar but when a hurricane happens, his boat gets on a anchor in a tiny colonial outpost in the Indian Ocean. During the night, a woman named Marie disappears. Jérôme Lange is in charge of the investigation.

The thing is, the investigation is not the main plot, as there are 5 different things happening in the island and you can investigate them (only for the questionnaire at the end of the game). But the main goal is decode the messages from the piano, the caves and a book just so you can open a jewel box, that gives you a code for the medallion. The medallion is the key that opens the cave at the end of the game.

The game is too cryptic and the codes needs to be written on paper to make sense of them, however even then it doesn't seem that someone is really able to figure them out. Most likely the solutions online comes from the developers themselves.