The people of Cumae worship Diana, goddess of hunting and chastity. Their rites however have been defiled by a nymph whose treachery has brought a curse on them. To propitiate the angry goddess, two faithful lovers must be sacrificed each year to a lake monster until a faithful lover can be found to offer his own life. Fidelity, therefore, is at a premium in Cumae, and victims are hard to find.
The plot is "part thriller about lovers being sacrificed to a monster, part burlesque sending up pseudo-classical and early romantic emotions".
A temple dedicated to Diana
Melibeo presides over preliminary rites on a day of sacrifice, assisted by Lindoro and Nerina, whose affair is coming to an end; Lindoro is tired of Nerina and hopes for a liaison with the shepherdess 'Celia'. Lindoro's sister Amaranta, recently arrived in Cumae, comes to worship. She is on the look-out for a lover but startled to hear of the risk in true love. Melibeo suggests that as High Priests are exempt, she might give her attentions to him. She agrees, on condition that he favours her brother's suit with Celia.
Perrucchetto, a traveller, philanderer and coward arrives claiming to have been chased by robbers. His racing pulse quickens when he sees Amaranta, to whom he swiftly declares love. She is also overcome, especially when discovering that he is a Count. Melibeo threatens Perrucchetto, who reacts by billeting himself on the High Priest.
Young shepherd Fileno laments the death of his beloved Fillide (Celia) killed by a snake. He is told by Nerina of Lindoro's desertion and she begs him to plead on her behalf; Fileno agrees (not realising that this is his beloved).
Celia arrives wearily with her sheep in search of her lover Fileno and sleeps amongst her flock. Nerina returns with Fileno, who, to his amazement and delight recognises Celia - alive and well. He is unaware of the fatal penalty awaiting faithful lovers, but Celia, spotting Melibeo waiting to pounce, spurns Fileno to save his life: naturally he is angry and desolate.
Fileno, intent on self-destruction, goes off followed by Celia, who is followed by Lindoro and Perrucchetto, who sees in Celia a more enticing prospect than Amaranta, who in turn is offended and turns back to Melibeo. Perrucchetto, rejected by Celia returns to make peace with Amaranta but then chases Nerina, infuriating Amaranta.
A dark wood
Melibeo tries to blackmail Celia into the match with Lindoro suggested by Amaranta - she must consent or else die with Fileno. Celia asks Nerina to warn Fileno that his life is in danger. Although Nerina agrees to help, as she has now fallen in love with Fileno her help is not altogether disinterested.
As the first act reaches its climax, Melibeo has Fileno tied up. Fileno curses Celia when he learns she is to marry Lindoro. At this point Nerina enters pursued by satyrs who carry off numerous nymphs, including Celia.
Celia is rescued by shepherds. Melibeo takes stock of the situation. If he could make a match between Nerina and Fileno that would leave Celia free for Lindoro and then he can claim Amaranta. He encourages Nerina to use her charm on Fileno and allows her to release him from his bonds. Fileno is at first grateful, but seeing Celia with Lindoro pretends ardent love for Nerina to spite Celia. She remonstrates with Nerina who advises that she forget Fileno.
Fileno resolves to stab himself, but first carves a message of love to Celia on a tree trunk. However in doing this he breaks his dagger, so determines instead to throw himself off a cliff.
As he is about to do this, the hunt assembles in honour of Diana. Perrucchetto enters pursued by a bear, followed by Amaranta fleeing a boar. Perrucchetto takes refuge in a tree; Amaranta faints just as Fileno kills the boar. When she comes round, Perrucchetto claims that he saved her, but the boar is borne off to the temple.
A dreadful grotto
Celia finds the message on the tree and seeks solitude in a cave. Melibeo, seeing this, changes his plans again: if Nerina can lure Perrucchetto into the cave with Celia they can be 'framed' as lovers and sent to the monster. This is accomplished and the pair are robed as sacrificial victims. Thunder proclaims the wrath of Diana.
A hall, then a landscape with a view of the lake
The victims take leave of their real lovers. At the last moment Fileno decides to sacrifice his own life to save Celia. As he offers himself to the monster, it transforms itself into Diana who accepts the purity and selflessness of his act and for ever absolves Cumae from the fatal curse. Apart from Melibeo, struck down by Diana's arrows, the opera ends happily with the union of Celia and Fileno, Amaranta and Perrucchetto, and Nerina and Lindoro.