Kazumi Kohinata is a major siscon for his younger sister Yuzu and her childhood friend Minatsu who calls him Kazu-nii and treats him as an elder brother despite their lack of family ties. One day his father announces that he and Kazumi's mother will be going to Las Vegas for a week along with Minatsu's parents and he must take care of both girls alone.

Kazumi now has one week to get Yuzu and Minatsu to fall madly in love with him.