The Fantasia in D minor, K. 397, was composed in 1782 or between 1786 and 1787 and left unfinished. (For this reason, the catalog lists it as a fragment). It remains unclear whether the composer had planned to expand the work, or imagined it as preceding a sonata or even a fugue (perhaps in the same key). Still, it is one of Mozart's most popular piano pieces.

In 1944, it was revealed that August Eberhard Müller (1767-1817), a German composer, organist, flutist, and conductor, wrote the Fantasia's last ten bars. Müller was a strong advocate of Mozart's music, and was highly esteemed by Beethoven.

Mozart's output in minor keys constitutes a tiny part of his production quantitatively. Only 14 of Mozart's more than 600 compositions are in D minor, including his final work, the Requiem. The emotional concentration encapsulated in pieces written in minor keys is unique, and Mozart's choice of D minor key for the Fantasia is meaningful.

The Adagio evokes the theatricality of an operatic scena, with many "changes of pulse and mood, [and] its startling silences and passionate outcries" [Zaslaw and Cowdery, The Compleat Mozart, 324.]. Chromaticism plays a prominent role in the thematic material throughout the Adagio, coloring the melodic lines and the descending bass figures in the left hand. However, Mozart resolves the tension in an arpeggiated major chord, signifying 'transcendence' over the weighty thematic material. —Enrico Elisi