Can someone please untie what on earth is happening with the harmony here for me please
. Did the music just modulate several times, or is he using a whole bunch of chromatic notes. You can tell my music theory sucks...balls 
I love music theory so I had a go at it!
He is doing a chromatic bassline, but everything is well within the confines of C minor.
I've simplified the score in the picture below. The upper system is the chords and the skeleton of the melody and the lower system is the chords in their root position with any missing notes (see analysis below) added. There is a Too Long Didn't Read explanation at the bottom of the post too

EDIT woops forgot to make the B in the first G major chord in the lower system a B natural

In the first bar, you have a D major 7 with a flat 9 added, with the 7th, C, as the bass note and the D tucked into the arpeggio figure in the bass. The dominant of c minor is G major. The dominant of G major is D major. Therefore I have analysed the first chord as the Dominant of the Dominant, with the 7th, C, as the bass note, and a minor 9th (Eb) added.
Analysing the first chord as the Dominant of the Dominant is very fitting because in the second bar comes the Dominant, G major, with the third, B natural, as the bass note. The bass note 7th (C natural) of the previous chord resolves nicely to the B natural.
The second chord in the second bar is a bit tricky, all you can tell at first glance is that it's a diminished chord (G dim). Diminished chords are often used in place of a dominant, so let's look where it's headed! In the third bar there is an F major chord with the third, A natural, as the bass note. The dominant of F major is C major. But wait, there is no C or E natural in the tricky chord, how can it be a C major chord?! However, there is a G, a B flat, and a D flat, which, if we also had the missing C and E natural, would make up a C major 7 with a flat 9 added, just like the D major in the first bar. We again get the 7th (B flat) as the bass note, again resolving to the third (A natural) of the next chord (F major). Therefore I analyse this chord as the Dominant of the Subdominant,
without the root note (C natural), with the 7th (Bb) as the bass note, and a minor 9th (Db) added .
Analysing it this way you get a nice standard fifths sequence D-G C-F in the first three bars.
The second chord in the third bar is just an f minor with the third, A flat, in the bass. f minor is the Subdominant of c minor, and F major is of course just the same subdominant, but in major. So the third bar is just a bar with the subdominant in it.
In the fourth bar you have a G as the bass note and a C and an E natural in the right hand. However I don't think it is a C major chord if you analyse its harmonic function. Since it has a G as the bass note and resolves in the next bar to a G major chord it should be analysed as the Dominant with a suspended 4th and 6th. In this case the 6th is an e natural instead of the expected e flat, but it resolves chromatically to e flat and then to D. Some chromatic stuff also happens in the left hand but it's just a nice effect that doesn't affect the harmonic skeleton.
In the last bar you are back in the tonic again of course.
TLDR:
C minor:
1st bar Dominant of the Dominant, D major 7 with minor 9th
2nd bar Dominant, G major, followed by the Dominant of the Subdominant, C major (missing the root and the third and also with a 7 and minor 9th)
3rd bar Subdominant in major, followed by subdominant in minor (F major and f minor)
4th bar Dominant with suspended 4th and 6th
5th bar Dominant
6th bar back to tonic
1st-3rd bar sequence D G C F, F being the subdominant
4th-6th bar dominant resolving to tonic