"Big brother, you're free today, right?" Miki, in a strangely good mood since morning, thrusts a bag containing her swimsuit in front of the protagonist. "Let's go to the pool together!" "That's sudden..." "Because it's summer vacation!" Miki flashes a carefree smile. She is a sister who loves her brother and always wants to be with him. They head to the pool, and just as they pass through the entrance— "Oh? Miki?" A familiar voice. Turning around, they see Miki's best friend, Reika, standing there. "R-Reika!?" Reika has the sharp eyes of a delinquent. However, the moment she sees the protagonist, her expression changes completely. "...So your brother is here too." "Y-Yeah!" "Heh..." Reika's gaze becomes fixed on the protagonist. "You're looking cool today too, big brother." "Huh?" "I-It's nothing!" Reika turns away, her cheeks flushing red. Meanwhile, Miki does not miss her friend's behavior. "...Reika." "W-What is it?" "You like my brother, don't you?" "—!" It was a bullseye. Then, Miki takes the protagonist's arm as well. "Big brother is playing with me!" "Huh!? Why do you get to decide that!" "Because I'm the one who invited him today!" "Then it's a competition!" "You're on!" And so, what was supposed to be a fun day at the pool— Turns into a strange rivalry between Miki and Reika over the protagonist. Who will have the last laugh: the sister or the best friend? The girls' heated battle at the summer poolside has begun.