Hiroki and Yukari had been a couple since they were students. Meanwhile, their juniors Tetsuya and Mei still hadn't developed to the point where they went out yet. However, inspired by Hiroki's marriage, they started dating and became lovers.

That was 3 years ago. Now. Hiroki and Yukari are happily married, Tetsuya and Mei are also planning their wedding. However, it seems that their juniors' relationship isn't so good lately.

After some probing, Tetsuya unexpectedly asks Hiroki to sleep with Mei. A surprised Hiroki decides to discuss it with Mei and ultimately chooses to accept Tetsuya's request. However, Tetsuya's request doesn't stop there...