遊覧船から転落した水原千鶴（桜田ひより）を助けるため、後先構わず海に飛び込んだ木ノ下和也（大西流星）は、どうにか千鶴の救助に成功する。だが、自身は意識を喪失。そんな和也のため、今度は千鶴が心臓マッサージと人工呼吸を施す。その甲斐あって、和也は無事に意識を取り戻すが、客である自分に人工呼吸までしてくれる千鶴に対し、和也の気持ちはまたしても混乱。2人が契約を解消する時は刻々と近づいているというのに、千鶴と出会ってからの出来事を一つずつ思い返す和也は、ついに、“返却不可”な自分の本当の気持ちに気づいてしまう…。
そんな中、和也は友人の栗林駿（鈴木康介）から彼女を紹介される。初めて会う駿の彼女の更科瑠夏（工藤美桜）は、千鶴に劣らぬ美少女だったが、和也はその顔に見覚えがあり…。
Kazuya Kinoshita (Ryusei Onishi) jumps into the sea to save Chizuru Mizuhara (Hiyori Sakurada) who has fallen from a pleasure boat, and manages to rescue her. However, he himself lost consciousness. For Kazuya's sake, Chizuru gives him a heart massage and artificial respiration. Thanks to that, Kazuya regains his consciousness safely, but Kazuya's feelings are confused again when Chizuru, who is a customer, even performs artificial respiration. Even though the time for the two to terminate their contract is getting closer and closer, Kazuya, who recalls the events since he met Chizuru one by one, finally realizes his true feelings of "non-returnable". …. Meanwhile, Kazuya is introduced to her by her friend Shun Kuribayashi (Kosuke Suzuki). Ruka Sarashina (Mio Kudo), Shun's girlfriend whom he meets for the first time, is a beautiful girl as beautiful as Chizuru, but Kazuya recognizes her face...
