Chopin wrote four impromptus -- a French word suggesting improvisation -- but affixed the word "fantaisie" (fantasy) only to the last one, perhaps implying a clearer form for the first three and a more rhapsodic nature for the last. In fact, though, it's in a fairly straightforward ABA pattern, with an unexpected twist in the coda. Chopin asked that this work, along with several others, be destroyed after his death (obviously his executors ignored him); it's speculated that he felt the piece was too derivative of the Op. 89 impromptu of Ignaz Moscheles. Chopin's manuscript carries the French inscription "Composed for the Baroness d'Este," which some people, notably pianist Artur Rubinstein, have interpreted to mean that the baroness commissioned the work for her exclusive use.

Following a dour, imposing opening note, the piece begins with fast, rippling figures in both hands. A second section, moving to D flat over left-hand triplets, offers a broad, lyrical melody with just a few touches of filigree near the ends of phrases. It serves as the basis of a couple of short quasi-variations, never straying far from the original theme. This melody, incidentally, was used about a century after its composition as the tune of the pop song "I'm Always Chasing Rainbows." The perpetual-motion opening section returns for what seems will be a fleet finale, but just before the end the music devolves into a swirling figure from which emerges a firm bass restatement of the second section's romantic theme, and the Fantaisie-Impromptu ends with a gentle arpeggio and a quiet chord.