The Unicorn (1955)

Harriet and Frank Allard have been married for many years. Almost twenty years ago, Harriet had a love affair with a colleague of Frank's, Claes von Klitzow, who made her pregnant. She had a son, Christer, whom Frank thinks that he is the father of. One day, Christer reveals that he is going to marry von Klitzow's daughter Louise. Harriet becomes upset but what will she do? Try to stop the marriage by telling the truth - which would endanger her marriage with Frank - or keep silent, which would mean that two half-siblings married?