Natsumi, a third-year ronin, felt inferior to his older brother Masaki in both looks and academic achievements. Unknown to anyone, he secretly harbored romantic feelings for Manami, his brother's girlfriend. One day, thanks to Masaki's arrangement, Manami began visiting Natsumi's home as his private tutor. Excited by the unexpected opportunity, Natsumi threw himself into their sessions with growing infatuation. Soon after, however, he learned that Masaki and Manami had become engaged. While Natsumi genuinely wished them happiness, the sounds of Manami's moans seeping from his brother's room began to twist his innocent heart, giving rise to dark, lewd fantasies.