Der Mann aus dem Bootshaus (1967)

Attractive Julia Swinton is a middle-aged woman. Her life went normally and without big occurrences. But through the unexplained death of a familiar young lady in a houseboat and the sudden appearance of a strange stranger, unrest and threat now break into the seemingly so peaceful everyday life of the Swinton family.
As it soon turns out, Mr. Swinton has a secret relationship with the lady on the boathouse...