Masato and Muraki watch Miyuki and Hiromi take a bath and later a black bikini catches their eye and the two of them are puzzled as to who it belongs to. Masato has tucked in the bikini bottom. When he later accidentally shows it in the presence of Miyuki, he catches his first slap in the face. But that's not all, because Masato's stepsister also appears, also called Miyuki, who lived abroad for a long time and has only now returned to Japan after six years.