Asa Mizushima, a transfer student visiting for a single summer, finds herself unexpectedly falling for the persistent and flirtatious Hajime Saisho. When she discovers that Hajime's childhood friend, Natsume Sorayama, harbors the same feelings, the two girls form a secret pact to share him. Together, they spend their final summer days indulging in a hidden, passionate relationship before their time together runs out.
Hajime navigates a risky and passionate summer as he maintains secret relationships with both Asa-chan and Nat-chan. Between daring encounters at school and intimate nights at home, the two girls compete for his affection. As the season draws to a close and the threat of separation looms, the trio shares a final, forbidden bond, cementing a secret that will tie them together long after summer ends.