Bokura no Isshuukan.

Until a month ago Tooru was an only child, but after his father got remarried, he found himself with two step-sisters, Hinaka and Hatsuna. While he gets along with Hatsuna, his relationship with Hinaka is—on the surface—full of constant bickering. In reality, they are in love with each other.

When their parents depart for a week-long honeymoon trip, Hinaka and Tooru get the chance to let loose while alone in the house. However, their precious time together is cut short when Hatsuna—who secretly harbors feelings for Tooru—catches them in bed!