The Voice of My Unfamiliar Little Sister (見知らぬ妹からの声 Mishiranu Imōto kara no Koe) is the 1st episode of the Kono Naka ni Hitori, Imouto ga Iru! anime series.
Shougo Mikadono, whose father Kumagorou Mikadono had just died, was hailed by someone who stood on the other side of the cementary house's wall. The caller, a girl, proclaimed to be his little sister, which confused Shougo greatly as he was an only child. The girl left, but not without leaving a cryptic message - one day, she would marry him.
Having returned from the suceesful training to become the head of the Mikadono Group, Shougo arrived at the airport to see his mother Kanoko Mikadono and her aid Risa Seri already there. It was revealed during the meeting that as a part of Shougo's agreements before, he would enroll to Miryuin Private Academy as per Kumagorou's wish, and found himself a potential wife among the students there.
In his first day at school (coincidentally also his birthday), Shougo met a girl named Konoe Tsuruma, and they shared the cream puffs that Konoe had bought from a bakery nearby. Shougo suddenly saw strange images as he faced Konoe, but quikcly dismissed it as a migrane. Shougo also explained to Konoe that he received a scar on his forehead after an accident as a child.
After Konoe is summoned to a student council meeting, Miyabi Kannagi, his quiet classmate who sat next to him, tried to tease Shougo, but he backed out of it. Later at night, he receives a birthday cake and an action figure of his favorite childhood show "Transforming Warrior Gunbarrion", by mail. After seeing a photo of him as a child being kissed by an unknown girl at an amusement park, he receives a phone call from his proclaimed "sister" confirming this, and revealed that the girl was her during childhood. Finally, she told him that they were in the same school now, and they would meet each others in face sooner or later.
The next morning, he ponders if the girl who chooses to marry will end up being his little sister. Then, at school, he caught sight of someone who wore the same wide-brimmed hat as the girl in the picture and was then waching him from the corner. Shougo chased after her and cornered the girl at the rooftop, who then told him "I'm curious about my older brother".