A normal, average Japanese teenager wakes up to a normal day. At the breakfast table, his little sister seems a bit more reserved than usual. At school, he finds that his childhood friend is a bit shy about talking to him. The class president barks at him rather harshly- more so than usual. Everything seems a little bit off. It isn't for a few days that he realizes that he's in an eroge- a very smutty one at that. People he's known for his entire life have been reduced to stereotypes. Needless to say, he doesn't take this very well. After a slight internal breakdown, the game's first choice is presented.
This is where the game's save system comes into play. You see, this game doesn't have a normal save system. Instead, it tracks which choice points you've encountered, and which choices you've taken. As the game progresses this becomes a large map, filling itself with your choices. At any time you can exit to this map and go to any other point in the game you've already been to. This is essentially an automation of what eroge players already do- make a map of choices and save at every opportunity. The catch is that the PC remembers his choices, and new ones appear as he gathers more intel. Thus, the player might want to go down a bad end in order to learn something.