The film's twist relies on a hidden connection revealed in the final act. Throughout the film, Jean-Louis speaks of his high moral standards and his search for a pure Catholic wife. He resists Maud, a divorcée, to save himself for Françoise.
However, it is revealed that Françoise was the secret mistress of Maud's husband. This was the affair that broke up Maud's marriage. The irony is devastating: Jean-Louis rejected the 'sinful' divorcée Maud only to marry the 'pure' woman who was the actual cause of the sin.
In the final scene on the beach five years later, when they run into Maud, Jean-Louis realizes this truth. However, he chooses not to confront it or acknowledge it openly. He sticks to his 'wager,' deciding to live within the comfortable lie of his marriage rather than face the messy truth, proving that his morality was always about appearance and safety rather than absolute truth.