Shortly afterwards Fra Diavolo arrives at the inn, disguised as the Marquis of San Marco, under which name the English lord has already made his acquaintance. He is not enchanted by the arrival of this Marquis; he fears a new flirtation with his own fair wife. Pamella wears most valuable diamonds, and these strike the eye of Fra Diavolo.
He sees that the English have been clever enough to conceal the greater part of their wealth and resolves to put himself speedily into possession of it.
He is flirting desperately with Pamella and looking tenderly at the pretty Zerline, when the soldiers return, having captured twenty of the brigands and retaken the greater part of Lord Cookburn's money and jewels. Lorenzo, the captain of dragoons is rewarded by the magnanimous Lord with 10,000 Lire, and may now hope to win Zerline's hand. But Fra Diavolo vows to avenge the death of his comrades on Lorenzo.
In the second act he conceals himself behind the curtains in Zerline's sleeping-room, and during the night he admits his two companions Beppo and Giacomo. Zerline enters and is about to retire to rest, after praying to the Holy Virgin for protection.—During her sleep Giacomo is to stab her, while the two others are to rob the English Milord.
But Zerline's prayer, and her innocence touch even the robbers, the deed is delayed, and this delay brings Lorenzo upon them. Fra Diavolo's two companions hide themselves, and the false Marquis alone is found in Zerline's room. He assures Lorenzo, that he had a rendez-vous with his bride, and at the same time whispers into Milord's ear, that he came by appointment with Milady, showing her portrait, of which he had robbed her the day before, as proof. The consequence of these lies is a challenge from Lorenzo, and a meeting with Diavolo is fixed. The latter is full of triumphant glee; he has arranged a deep-laid plan with the surviving members of his band and hopes to ensnare not only Lorenzo but his whole company. Ordinarily Diavolo is a noble brigand; he never troubles women, and he loads poor people with gifts, taking the gold out of rich men's purses only, but now he is full of ire and his one thought is of vengeance.
Finally he is betrayed by the carelessness of his own helpmates. Beppo and Giacomo, seeing Zerline, recognize in her their fair prey of the evening before and betray themselves by repeating some of the words which she had given utterance to. Zerline, hearing them, is now able to comprehend the wicked plot, which was woven to destroy her happiness. The two banditta are captured and compelled to lure their captain into a trap. Diavolo appears, not in his disguise as a Marquis, but in his own well-known dress, with the red plume waving from his bonnet, and being assured by Beppo, that all is secure, is easily captured. Now all the false imputations are cleared up. Milord is reconciled to his wife and Lorenzo obtains the hand of the lovely Zerline.