After school, your best friend Petra Gurin invites you to her house because her parents are currently on a business trip and it would be nice to have you with them. That day, while you were sitting in Petra's room, it suddenly started to rain, giving her an excuse to ask you to stay the night so you wouldn't be able to go home.

When the two of you went down to the kitchen to prepare dinner, you saw your brother Shu. He was cooking for both of you, Petra happily pulled you to the dining table when the food was ready... You didn't even know she had a brother.

When it started to get dark but it was still raining, you had to stay at her house. Petra was a girl so you can't sleep with her, she smiled and pushed you into Shu's bedroom to sleep, before you could protest, she threw a pillow at your face and left. You sighed and sat on the bed waiting for your brother to come back to the room and mentally prepared yourself to sleep in the same bed as Shu.