Bastienne, a shepherdess, fears that her "dearest friend", Bastien, has forsaken her for another pretty face, and decides to go into the pasture to be comforted by her flock of lambs.
Before she can leave, however, she runs into Colas, the village soothsayer. Bastienne requests the help of his magical powers to help win back her Bastien. Colas (being a soothsayer) knows all about the problem, and comforts her with the knowledge that Bastien has not abandoned her, rather, he's merely been distracted lately by 'the lady of the manor'. His advice is to act coldly towards Bastien, which will make him come running back.
Bastien is heard approaching, so Bastienne hides herself. Bastien swaggers in, proclaiming how much he loves Bastienne. Colas informs him that Bastienne has a new lover. Bastien is shocked and asks the magician for help.
Colas opens his book of spells and recites a nonsense aria filled with random syllables and Latin quotations. Colas declares the spell a success and that Bastienne is in love with Bastien once more. Bastienne, however, decides to keep up the game a bit longer and spurns Bastien with great vehemence. Bastien threatens suicide, which Bastienne merely shrugs off.
Finally, the two decide that they have gone far enough and agree to reconcile. Colas joins them as they all sing a final trio in praise of the magician.