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Mika, the younger sister, and her husband Shun move into the apartment next door to where her older sister Ryo and her husband live. One day, Mika sees Ryo bringing a man home and goes to Ryo's husband Yuichiro to discuss it. However, Yuichiro remains calm even after hearing her story, and Mika ends up embracing him and playing with him. Then, by chance, Ryo finds out about her relationship with Yuichiro, and in retaliation, she seduces Shun. Before long, the sisters and their wives start swapping partners and having sex...