Mozart was a seasoned though not yet distinguished opera composer with one such work behind him when, at age 12, he was asked to write a pleasant singspiel to be performed in the tidy Rococo garden theater of Dr. Anton Mesmer, the pioneer hypnotist who would lend his name to the term "mesmerism." The story can be traced back, through some detours, to a work whose text and music were written by French philosopher Jean-Jacques Rousseau, Le Devin du village (The Village Soothsayer). This was a folk-ish tale of a shepherd and shepherdess interacting with a village sage and supposed magician. The story went through several hands, including those of a hack named Harny, who in 1753 dumbed down Rousseau's arcadian idyll into a coarser (and frankly more realistic) tale of country bumpkins. Harny's text was translated into German by Friedrich Wilhelm Weiskern, and this libretto, revised by a Salzburg court trumpeter named Andreas Schachtner, is what came into young Mozart's possession.
The brief overture is notable for offering a foretaste of the theme Beethoven would employ at the beginning of his "Eroica" Symphony nearly four decades later. Otherwise, the score generally adheres to the sing-song style of German folk music through the course of 11 little arias, two duets (plus a third in the form of a recitative and arioso), and a trio finale, all performable, with dialog, in well under an hour. The arias take the cavatina form, each in two parts contrasting in mood and meter. Only in the closing trio does Mozart even hint at the talent he would later show for ensemble numbers, and even this has more in common with Mozart's basset horn pieces than with Così fan tutte.
Even so, the score is not exactly rudimentary. Mozart creates a rustic sound with parallel thirds and sixths, and he introduces the soothsayer, Colas, who carries bagpipes, with musette-style music containing bass fifths and some raised fourths. And Colas' hocus-pocus witches' chorus (No. 10) is a stormy, minor-key episode of some imagination.
Bastien und Bastienne is charming enough and easy enough to provide fodder for school productions (and is a welcome, light change from the depressing English girls'-school standby, Dido and Aeneas), but it would frankly merit little attention today were it not associated with a pre-teen named Mozart.