妹こうかん！

While their parents are out of town for a few days, best friends Takamichi and Kousuke agree to swap their younger sisters. It starts out as a joke, but surprisingly, both sisters agree to the plan, and the trade becomes a reality.

Takamichi's outgoing and popular sister Akari secretly has feelings for her brother, but she tends to tends to act in the opposite direction of her feelings. She leaves home to go live with Kousuke.

Meanwhile, Kousuke's shy and awkward sister Ayane temporarily moves in with Takamichi. While she is usually very reserved, she boldly crawls into his bed at night. "If I don't do this, I won't be able to sleep."