Since Acis and Galatea has been adapted many times, it is impossible to provide a single synopsis that accurately reflects every presentation of the work. The following is a synopsis for the typical two act presentation of the work that is most often used for modern performances.
Shepherds and nymphs enjoy "the pleasure of the plains". Galatea, a semi-divine nymph, is in love with the shepherd Acis, and tries to hush the birds that ignite her passion for him ("Hush, ye pretty warbling quire!") Acis's close friend, the shepherd Damon, provides counsel to the lovers as they pursue each other. He sings a beautiful siciliana-style serenade, "Love in her eyes sits playing", upon their first meeting. The act closes with a duet by the young lovers, "Happy we", which is echoed by a chorus (not in the Cannons original).
The opera shifts from its pastoral and sensual mood into an elegiac quality as the chorus warns Acis and Galatea about the arrival of a monstrous giant, Polyphemus, singing "no joy shall last". The fugal minor-key of the chorus's music along with the percussive lines in the lower instruments, indicating the heavy footsteps of the giant, provides an effective dramatic transition into the more serious nature of the second act. Polyphemus enters singing of his jealous love for Galatea, "I rage, I melt, I burn", which is in a part-comic furioso accompanied recitative. This is followed by his aria "O ruddier than the cherry" which is written in counterpoint to a sopranino recorder. Polyphemus threatens force but is somewhat soothed by the impartial shepherd, Coridon ("Would you gain the tender creature"). Meanwhile, Acis ignores Damon's warning of the fleeting existence of love's delight ("Consider, fond shepherd") and responds hostilely with the determination to resist ("Love sounds th' alarm"). Acis and Galatea promise eternal fidelity to each other in what begins as a duet ("The flocks shall leave the mountains") but ultimately turns into a trio when Polyphemus intrudes and brutally murders Acis in a rage. Galatea, along with the chorus, mourns the loss of her love ("Must I my Acis still bemoan"). The chorus reminds her of her deity and that with her divine powers she can transform Acis's corpse into a beautiful fountain. The opera closes with Galatea's larghetto air, "Heart, the seat of soft delight", where she exerts her powers to enact the transformation, ending with the chorus celebrating Acis's immortilization.