Toshi Makishima's father Kenzou is leaving on business trip. Unfortunately for Toshi his father's new wife "Clara" is a rowdy American woman that he can't stand. 

Toshi has to deal with being her slave even though he despises it. He can't help it due to her being ostracized by the local Japanese housewives. She's frustrated by that and spends her days drinking beer. 

When Toshi comes back with the beer Clara requested he finds her asleep with a vibrator inside her. Unsurprisingly he tries to do something about it, but in turn makes Clara orgasm in her sleep. Causing Clara to want to have sex with Toshi. 

She mentions early that she's unsatisfied with Kenzou and this is when Toshi finds out it's because he doesn't have a big enough dick for her. Toshi is luckily well-endowed. This jump starts their stepmother son sexual relationship.