Gilberte Valandray has already been married once before, to an American called Eric Thomson. However, the marriage wasn't a very happy one; her husband never allowed her to kiss him "on the mouth". And since their wedding was never certified by the French consul, it has no legal value in France.
On returning to Paris, Gilberte has married Georges Valandray with whom she is perfectly happy. Georges, who believes in marital bliss as long as a man is his wife's first and only husband, knows nothing about her marriage to Eric Thomson. Only Gilberte's sister, a confirmed spinster, knows the secret.