Constance is in Paris to study, her parents being market traders in Orléans. There is a piano in the flat that's strictly off limits to Constance, which is a pity since she plays. We learn that her music teacher in Orléans discouraged her, thus her attempt to study academically, a forlorn attempt as it turns out when she fails her examinations. We also learn that Henri's wife, long deceased, played, thus Henri's sensitivity on the matter. During a visit by Henri's son, Paul, his wife, Valerie, asks Constance to play the piano. Henri relents in the circumstances but as Constance plays Henri is moved to tears, but he realises that Constance has talent.
Henri doesn't like Valerie and plots a devilish scheme to get Paul to realise that she isn't the woman for him. Henri asks Constance to seduce Paul. Not to the point of anything serious happening, but to make him realise that there are better choices for him. Constance doesn't want to do this, but a bit of less than gentle persuasion by Henri, along the lines of "if you don't do it you must leave', causes Constance to play along with the scheme. It turns out to be a bit too successful as Paul falls for Constance, while his wife leaves him and starts divorce proceedings.
We start to see Henri taking an interest in Constance's wellbeing, in fact being quite paternal, and he also take steps to encourage her to pursue her music. Meanwhile, Paul's short infatuation with Constance seems to have elevated his hormones, because we next learn that his wife is pregnant, prompting a reconciliation, as they had been trying for a child for a long time.
Henri has also worked a bit of magic that ensures that Constance is selected to audition for the London School of Music. Everything in fact is working out fine. Is Constance successful? I'll leave it to you to find out.