Mary, whose husband has passed away, lives with her son from her previous marriage. She has a faint romantic feeling for her son, who reminds her of her late husband. They go on a trip to a hot spring just the two of them... though they are legally father and son, they are complete strangers. Even though they know that this is wrong, a forbidden bond binds them together. Unable to resist the pleasure of immorality, the two make love in the night in Joshu... they can't take it anymore. The moans of father and son echo endlessly through the darkness of the night.