College freshman Yuuma hopes for a fresh start, but his life takes an unexpected turn when the popular Nana-senpai recruits him to be her "sex friend" during a welcome party. Despite his nervousness and virginity, Yuuma is quickly seduced by the bold senior at his apartment. As they establish a casual physical relationship, Nana decides to keep certain secrets about her personal life to ensure their arrangement continues without the complications of romance.
Still reeling from their first encounter, Yuuma is surprised when Nana-senpai visits him at work and invites him to a love hotel. As they explore new heights of intimacy, including Yuuma's first kiss, he finds himself falling deeper for the older girl. Despite the knowledge that she is likely unavailable, Yuuma becomes addicted to the thrill of their secret, physical relationship.