The game places the player in the role of Ichihiko. At first glance, he is a fairly ordinary young man who lives an (almost too) ordinary life. He lives at home with his two sisters, Mizubashou and Shirotsumegusa, constantly gets into arguments with his next door neighbor Hinagiku, goes to school with all three and spends the days enjoying life in a virtually perfect world.
There are, of course, some oddities. Firstly, the only humanoids in the world seem to be Ichihiko, his older sisters, Hinagiku and their teacher, Kobatomugi-sensei. Everyone else looks like something resembling old sci-fi movie drone robots that communicate by repeating the phrase "oui monsieur" known as saberhagen-class droids (and are understood the same way Star Wars characters understand the beeps coming from R2-D2). This is a world where it never rains. Where weather conditions are always convenient. Where Ichihiko's favorite shows are always on TV and his favorite foods are always served. Yes, this world is clearly fake.
As the story progresses, the player discovers that Ichihiko is actually the "last mankind" (in other words, the last human left in the universe). He was found in a stasis capsule by the robots that inhabit the spaceship Saint Exupéry-go. Feeling that he is too young to govern over the ship and perform his role in bringing about the second coming of humanity, the robot R-Mizubashou proposes that he be awakened and raised by them, and that a suitable "happy" world be built just for him; a world where he has a loving family, lots of friends and feels no hardships or pain.
Having been without any humans to serve for nearly a millennium, the robots draw upon on almost every possible source to build what is to be Ichihiko's perfect world. To this end, they have used data from "older" robots on board, books, movies and even manga. While the majority of the robots involved willfully continue the ruse (and do everything they can to dispose of dissenters), there are some who wish to tell the "last mankind" the truth.
[Taken from a review by Moroboshi Yuumei]