元カノの七海麻美（秋田汐梨）とレンタル彼女の水原千鶴（桜田ひより）が同席する羽目になった飲み会で、すっかり酔っ払ってしまった木ノ下和也（大西流星）は、麻美から優しく介抱されるうちに強い未練を自覚し、性的欲求を抑えきれなくなる。ところが、麻美が千鶴を悪く言うのを聞いた途端、今度は千鶴の肩を持ち麻美をいさめてしまう。本命は麻美、でも千鶴を傷つけたくはない…そんな混乱状態の和也に対し千鶴は、和也が予想もしなかった言葉をかける。
後日、和也は麻美を含む大学の仲間と出かけたビーチで、地味な“一ノ瀬”姿の千鶴と遭遇する。幸い誰も一ノ瀬が和也の“彼女”の水原千鶴とは気づかないが、水着姿の麻美に興奮して今日こそ麻美の気持ちを確かめようと意気込んでいた和也はパニックに。その成り行きから、和也はトイレの個室に千鶴を引っ張り込んでしまうのだが、やってきた麻美がその個室のドアを開け…！？
Kazuya Kinoshita (Ryusei Onishi) gets completely drunk at a drinking party where his ex-girlfriend Asami Nanami (Shiori Akita) and rental girlfriend Chizuru Mizuhara (Hiyori Sakurada) are together. As she gets older, she becomes aware of her strong regrets and becomes unable to suppress her sexual desires. However, as soon as he heard Asami say bad things about Chizuru, he took Chizuru's shoulder and scolded Asami. Her favorite is Asami, but she doesn't want to hurt Chizuru. Later, when Kazuya goes to the beach with his university friends, including Asami, he encounters Chizuru in plain "Ichinose" clothes. Fortunately, no one realizes that Ichinose is Kazuya's girlfriend, Chizuru Mizuhara, but Kazuya, who was excited by Asami in a swimsuit and was determined to check Asami's feelings today, panicked. As a result, Kazuya pulls Chizuru into a private room in the bathroom, but Asami arrives and opens the door...! ?
