There are a number of famous passacaglias by Shostakovich, who tended to use it to create powerful laments. This one is an expressive duet for the violin and cello over a set of chords in the piano that repeat six times. Here is that progression:
This progression outlines two tetrachords (groups of four notes). The first is in the tonic key of B flat minor and has the notes B flat, F, G natural and A natural. The A natural would normally lead us back to the tonic B flat, but Shostakovich follows it with a second tetrachord F#, G natural, A natural, B natural which suggests a modal E minor, the key of the first movement and the work as a whole.
Incidentally, the B natural that the progression ends on is in a Neapolitan relationship with the tonic: a semitone higher (which we would easily see if it were spelled C flat). Where else have we seen a Neapolitan relationship? Oh yes, in the second movement scherzo where the contrasting middle section was in G major, a Neapolitan relation to the tonic F# major. A very famous use of this kind of relationship is in the transition from the first to the second movement of Beethoven's late C# minor quartet. The first movement is in C# minor and the second in D major, the Neapolitan.
Here is the melody as it first appears in the violin. It is quite a long one, 45 seconds long, so requires three lines:
Some features: it begins with an upbeat fifth to tonic leap, just as the scherzo did. From there it mostly moves by step and features a mournful turn figure that comes three times. The movement unfolds with variations on this melody in both violin and cello and ends with a B natural pedal and outlining the chord of E minor.
Let's listen to that performance by Argerich, Kremer and Maisky: