Three years ago, Minako lost her husband and has struggled to recover since. Recently, she started attending a cultural school, gradually regaining her smile, which brought relief to her son, KENJI. One day, KENJI accidentally discovers that the cultural school his mother attends teaches "kinbaku" bondage classes for housewives. He catches sight of his mother's helpless state and the rope marks left on her body. Overcome by impulse, KENJI ends up tying up his own mother...