Gaston wants more than anything to marry the naive Simone. But where he is, there is Sidonie, a local singer romantically pursued by the police commissionaire.

Rather than commit suicide, it is better to stay at the clinic of Professor Patureau-Duparc. This practitioner experiments on his clients with a serum that modifies their personality. Thus Yolande and Albert become new Romeo and Juliet. They end up marrying each other after having regained their primitive selves. Their married life is then a hell and they welcome with joy the proposal of a new injection of serum.