Amos Vogel in Film as a Subversive Art:
A censorship landmark case: the entire plot pivots on an act of intercourse, during which the woman accidentally discovers the vital clue to the film’s mystery. The complete absence of nudity and total relevance of the scene to the plot posed an impossible problem for the American censors, and led, upon appeals against its prohibition, to the abolition by the Supreme Court of the entire system of American state censorship in 1967. This development contributed significantly to the later era of sexual permissiveness in the American cinema.
Snippet from NY Times (Spoilers within link):
In a most restrained way, [A Stranger Knocks] tells a story of a young and pretty woman who lives alone in a house by a lake in the country. One day a strange man comes by and, seeing the lonely situation, invites himself in out of the rain. The woman is quietly hospitable and allows him to remain overnight. He stays on the next day and the next day. Pretty soon the woman begins to fall in love.
During this period of muted courtship, the woman painfully reveals that she is an unappeased war widow. Her husband, who was with the underground, was captured, tortured and finally executed by one of his own countrymen. She cannot get over the horror of it or the memory of her love. The man, a pleasant, stolid fellow, tells little about himself.
Subtitles:English – srt