Note: Only a half-hour fragment from The Prude's Fall survives, and the first paragraph of this synopsis is based on contemporary reviews.

Haughty society lady Beatrice Audley tests the devotion of her French fiancé Captain André le Briquet by calling off their engagement and introducing him to her opposite in temperament, Sonia Roubetski. André soon proposes to Sonia, upsetting Beatrice's plan. On their wedding day Sonia hands Beatrice a letter to give to André confessing her involvement in a gambling racket; but he receives it only after the ceremony, and turns on both his bride and his ex-fiancée.

André asks Beatrice whether Sonia intended him to receive the letter before the ceremony, and accuses her of setting him up. Just as he vows to forgive his new bride, she commits suicide by poisoning herself, dying in André's arms. He swears vengeance on Beatrice.

Two years later, André returns from abroad to take up a military attaché role in the French embassy. He arrives at Carey Abbey, the home of Dean Audley and his wife, where Beatrice and her cousin Sir Neville Moreton are staying. Neville instantly distrusts André, but Beatrice is more forgiving. Soon they are being talked about in the society papers, angering Neville.

André asks Beatrice to "come to me", which she interprets as a proposal. She is taken aback when he tries to persuade her to share his view of marriage as "an unnecessary formality", but admits her love for him. He tells her to go to the home of Laura Westonry, where he will return to hear her answer a few days later. Beatrice, it transpires, had once snubbed Laura precisely for running away with a man without marrying him. Laura shames her by agreeing to help.

Sir Neville, meanwhile, hears unpleasant gossip about André at his London club, and decides to return to the country to intervene. His car breaks down on the way, but he arrives at Laura's house before André, and tries to persuade Beatrice to give up her plan to become his mistress. André appears before she has made her mind up, and gets into an argument with Neville, who tells him to leave, but grants him five minutes alone with his cousin.

Laura then thanks André for a letter [which Beatrice has seen but the audience hasn't - the earlier scene with Beatrice and Laura seems to have been cut] which appears to explain events prior to their first engagement. Beatrice, also responding to the letter, apologises to André for testing him in the first place; and he in turn admits that he has been testing her. André says that his feigned love has become real, and they kiss.

Meanwhile Laura, from an upstairs window, pleads with Neville to stop persecuting André. He re-enters the house and asks André to come clean. But it is clear that he and Beatrice are committed to their course of action, and Neville backs down. To Laura, and then to his driver, he admits bafflement at the ways of women, and returns to London.