A woman has an illegitimate son. She used to be a music-hall artist (les Folies Bergère) and she has had a lover (a politician) for many years. Now the child has become a young man and the politician's career is buoyant: he is to become a State secretary. This nasty man is sick and tired of his lover's friends (among which Michel Simon in a cameo). The lady's son, who is a journalist, decides to blackmail the reluctant father-in-law.
Like many other early ventures in sound film, Illegitimate Child was an adaptation of a stage play. This was the first fully talking picture to be made in a French studio. In addition to the technical problems presented by the heavy new sound cameras, L'Herbier was also required to make the film simultaneously in three different language versions (French, English and German) which meant that several actors had to be used in some of the roles. The film was sufficiently successful to attract other similar offers.