"I want to become a man who can walk proudly by her side." The protagonist, Kosuke Takahara, is in love with his childhood friend Suzume Azumino, who is one year older than him. Because their families are close, the two of them have long maintained a relationship like "siblings." However, determined to put an end to that dynamic this summer, Kosuke has decided to confess his feelings during a joint family vacation. Then comes the first day of the trip, and an unexpected problem strikes. Due to a booking error, they find themselves one room short. Resigning himself to the situation, Kosuke offers the room to Suzume and prepares to share a room with his parents. However, Suzume speaks up. "Is it not okay for Ko-kun and me to share a room?" Kosuke is flustered by the proposal, thinking that even for childhood friends, a young man and woman sharing a room is... But in the end, Suzume wins him over, and they end up sharing a room. "...I guess Suzu-nee really doesn't see me as a member of the opposite sex after all..." Seeing how Suzume doesn't seem the least bit bothered by the arrangement, Kosuke quickly grows anxious about how things will go. But he did not know. That Suzume, the one who suggested sharing the room, had also resolved to confess her love to Kosuke during this very trip...