A calm spring day. The protagonist awakens in a hospital bed. However, he cannot remember at all who he is, or why he is laying in a hospital bed. Confused, he tries looking for clues to discover his identity, when four girls appear before him.
Kaede Takanashi, a girl who refers to herself as the protagonist’s little sister; Ayana Sakura, who is said to be a close friend of Kaede, as well as a close friend of the protagonist; Aoi Hazuki, who claims, “I am the real little sister here”; and Mashiro Hinoue, who did not say anything detailed, but calls the protagonist “big brother.”
Among the girls that showed up, three of them claim to be the protagonist’s “little sister,” but the three do not seem to be related themselves. Who is the real little sister, and why are they all claiming to be his little sister? The amnesia-stricken protagonist has not a clue.
The confused protagonist looks to his side and sees an unfamiliar smartphone. “Is this my smartphone?” When he powers it on, the screen suddenly lights up, and four girls he has never met project into the room before him. “Good afternooon!! We are the Little Sisters!!” The girls insist that they are also the protagonist’s little sisters, half-jokingly saying, “Big brother is surrounded by no one but little sisters, isn’t he!” The Little Sisters say, “We get that big brother lost his memories, but we don’t get how it happened…” and “We’ll lend big brother our strength to get back his memories!”
Following the girls suggestion—“First, let’s figure out who the real little sister is. If we look at the last name on the name plate of the hospital room, the real little sister should become clear”—the protagonist checks the name plate, but then—.
