Third Movement: Presto agitato, 4/4 time, C-sharp minor

Sonata form. From the beginning of the movement, the music rapidly builds in intensity through the restless motion of non-melodic arpeggios (referred to as A), gradually narrowing sf accents, and harmonic changes. Following a dominant pedal point, the transition section begins at measure 11. From measure 21, a melodic theme (referred to as B), contrasting with A, appears in the dominant key of G-sharp minor. However, the expected tonal stability of the subordinate theme is low, and the bass line soon descends chromatically, moving away from G-sharp minor. From measure 43, where G-sharp minor becomes clear and stable again, the continuous sixteenth-note motion, which had been almost uninterrupted since the beginning of the movement, disappears, and the eighth note becomes the standard rhythm. Simultaneously, the texture changes to a contrasting succession of homophonic chords. The codetta (from measure 57) is based on B.

In the short development section, after themes A and B appear in the same order as in the exposition, a motive derived from the melody at measure 21 is varied over a dominant pedal point on G-sharp, the dominant of the movement's tonic key, harmonically preparing the recapitulation. The recapitulation concisely prepares the tonic restatement of theme B by omitting the transition section.

The structure of the coda (from measure 159) corresponds to the development section in aspects such as themes A and B appearing in the same order as in the exposition, but improvisatory passages are inserted without warning between the previously stated themes. Here, one can point to the 'fantasia-like' characteristic suggested by the title. As the rapid passages decrease in speed and reach the dominant via a bass ascent of a second (a semitone ascent in the coda), corresponding to the end of the development section (measures 100–101, coda measures 188–189), the codetta's musical idea reappears with the resolution to the tonic chord.