wife, Lord and Lady Allcash, at the inn of Terracina, kept by Matteo, whose daughter Zerlina is loved by Lorenzo, a young soldier, on the eve of starting to capture Fra Diavolo when the action of the opera begins.  In the first scene the English couple enter in great alarm, having narrowly escaped the robbery of all their valuables by Fra Diavolo’s band.  The bandit himself, who has followed them on their journey in the disguise of a marquis, and has been particularly attentive to the lady, enters the inn just as Lord Allcash has been reproving his wife for her familiarity with a stranger.  A quarrel ensues in a duet of a very humorous character ("I don’t object").  Upon the entrance of Fra Diavolo, a quintet ("Oh, Rapture unbounded!”) ensues, which is one of the most effective and admirably harmonized ensembles Auber has ever written.  Fra Diavolo learns the trick by which they saved the most of their valuables, and, enraged at the failure of his band, lays his own plan to secure them.  In an interview with Zerlina, she, mistaking him for the Marquis, tells him the story of Fra Diavolo in a romanza ("On Yonder Rock reclining"), which is so fresh, vigorous, and full of color, that it has become a favorite the world over.  To further his schemes, Fra Diavolo makes love to Lady Allcash and sings an exquisitely graceful barcarole to her ("The Gondolier, fond Passion’s Slave"), accompanying himself on the mandolin.  Lord Allcash interrupts the song, and the trio, “Bravi, Bravi,” occurs, which leads up to the finale of the act.  Fra Diavolo eludes the carbineers, who have returned, and they resume their search for him, leaving him unmolested to perfect his plans for the robbery.

The second act introduces Zerlina in her chamber about to retire.  She first lights Lord and Lady Allcash to their room, a running conversation occurring between them in a trio ("Let us, I pray, good Wife, to rest"), which by many good critics has been considered as the best number in the work.  Before Zerlina returns to her chamber, Fra Diavolo and his companions, Beppo and Giacomo, conceal themselves in a closet, and, somewhat in violation of dramatic consistency, Fra Diavolo sings the beautiful serenade, “Young Agnes,” which had been agreed upon as a signal to his comrades that the coast was clear.  Zerlina enters, and after a pretty cavatina ("’Tis to-morrow”) and a prayer, charming for its simplicity ("Oh, Holy Virgin"), retires to rest.  The robbers in attempting to cross her room partially arouse her.  One of them rushes to the bed to stab her, but falls back awe-stricken as she murmurs her prayer and sinks to rest again.  The trio which marks this scene, sung pianissimo, is quaint and simple and yet very dramatic.  The noise of the carbineers returning outside interrupts the plan of the robbers.  They conceal themselves in the closet again.  Zerlina rises and dresses herself.  Lord and Lady Allcash rush in en deshabille to find out the cause of the uproar.  Lorenzo enters to greet Zerlina, when a sudden noise in the closet disturbs the company.  Fra Diavolo, knowing he will be detected, boldly steps out into the room and declares that he is there to keep an appointment with Zerlina.  Lorenzo challenges him, and he promises to give him satisfaction in the morning, and coolly effects his escape.  One of his comrades, however, is captured, and to secure his own liberty agrees to betray his chief.