The player is on a southern island called Suzunari Jima. The 15 girls from Seika Joshi high school are also on the island for summer vacation.
The game starts as the master of the hotel tells a story of a hidden treasure on this island. Since many of the girls are pretty bored on the island, they decide to go look for the treasure.

Each day, the girls break up into several groups, and go looking for clues in finding the treasure. There are ten different locations on the island, and there are three times during the day (morning, noon, afternoon) that the player/girls can go to these locations. Then during dinner, everyone talks about what happened that day and makes plans for the next day.

[from Hitoshi Doi's site]