Mozart’s Fantasy in D minor, K. 397, is one of his most popular and accessible works for the piano. It may surprise you to learn that Mozart left it unfinished (his manuscript stops on a dominant 7th chord in bar 97), and that the ending we all grew up with was probably finished by August Eberhard Müller. Scholars believe Mozart might have intended to write something else in conclusion, possibly a fugue, but mystery still surrounds his plans for the work.
The idea of a fugue is a distinct possibility as there is a precedent, the Fantasy and Fugue in C minor, K. 394, written around the same time (1792). Here it is (with rolling score) played by Gianluca Cascioli in a performance that shows great attention to the composer’s articulation markings but without sounding at all dry or pedantic. If you are looking for an interesting alternative to a Bach Prelude and Fugue, you might want to consider this piece.
Returning to the D minor Fantasy, many listeners are startled to hear what Mitsuko Uchida does at the point Mozart left off. Rather than finish with the traditional ending (we can’t be 100% sure it was by Müller, but we know it wasn’t by Mozart), she feels justified in coming up with her own ending. And fascinating it is too (listen from 5:50)!
If you would like to delve further into the story of the D minor Fantasy, Ephraim Hackmey’s thesis is well worth reading.
I have made my own walkthrough of the piece for the Online Academy, giving suggestions for pedalling, articulation and practice. Here is a short excerpt, focussing on the Adagio.