Slave Ship (2010)

Onimata Kan (Tarô Suwa) is a middle-aged writer of BDSM literature fame and he hosts in a Tokyo Bay houseboat named the "Slave Ship" swinger-style BDSM parties attended by couples. When he pays a visit to the Umezu onsen owned by a couple who are frequenters of the Slave Ship, he learns that the elderly wife Kikue (Kyôko Aizome) has moved to a mental institution. Her husband Zenzaburo- claims that she is suffering due to abnormal interest of Kazuo Kitagawa, a married young admirer attending the Slave Ship parties. Onimata Kan is intrigued and finds Kitagawa, learning from him the intimate aspects of his dangerous liaison with Kikue. imdb