Perhaps the first interesting aspect of this song that you'll notice is the rhythm.
But it's also interesting melodically and harmonically, with a leading melody
that's like an over-eager dog pulling a
lagging harmony along. Let's look at each bar in turn.
A barely noticeable leading-tone effect, 7-1'. On first hearing, it could be 3-4 (or any other semitone), but the likeliest interpretation is 7-1'. And as the song goes on those notes still sound in the memory and their interpretation as 7-1' is strengthened. And then there's the replay value of the song, so first hearing is arguably an unimportant corner case.
Now the harmony (I) supports the 1 sustained from bar 1 and also the 5-3 that follow. That 3 sustains into the 5 of vi7, then comes another 1 which is 3 of the sustained vi7. Just before we end the bar we have a brief 2 which is the sus4 of vi7 and therefore causes a bit of disagreement and interest. Try playing the vi7 again through bar 3 while sustaining that 2: how odd does that sound? The melody is writing checks that the harmony has to cash otherwise the flow breaks and things fall apart. Or try moving back to I instead, which makes the 2 want to move to 1.
Even though it was the melody that caused the disagreement between vi7 and 2, it's the harmony that capitulates, moving a fifth down to ii7 so that the sustained 2 becomes its 1. Then there's a 4, which is 3 of ii7. The ii7 resolves down another fifth to V7 where the sustained 4 is its 7. And we end this bar with another leading-tone effect 7-1'.
Bar 4 is very similar to bar 2 except it ends on a brief 5, turning the vi into a vi7, which you would think would want to resolve to ii. Moving to ii does sound ok, but
that would just be repeating the cycle and failing to develop the song.
Instead, we move to iii where the sustained 5 is its 3. There is no melody note for the first half of this bar, and this phrase is
melodically an abbreviated form of the one in bar 3 ("abbreviated" because this time we skip the 3 of the chord and go straight to its 5, and even that is played in the bar
before this one). We might expect to hold that iii for the rest of the bar. Doing so sounds appropriate, but rather predictable compared to what actually happens, which is a mid-bar change to I7, which feels like it's hurrying things along even though there's no change in harmonic tempo. In the change from iii to I7, you can hear the 5 of iii (which is 7 of the key) descent a sweet min2 (which turns
the iii into a dim5 chord, and the iii embedded in every dominant 7 chord is a dim5 chord) to become the b7 of I7. In the melody is a repeated 5 then 1'-6. That 6 is interesting:
the I7 is promising a resolution to IV but, before that happens, the melody steals the harmony's thunder by playing 6, which is 3 of IV and exactly what that b7 in the I7 wants to resolve to (and now we've had a chromatic descent 7-b7-6). So now we have a little disagreement, which could go either way. For example, the harmony might move to V7, and so insist that the melody drop back to 5. For fun, try
playing I7-V7-IV-vi (instead of I7-IV-V7-vi), which simply flips the middle 2 chords of the original.
Here's how that looks, with the corresponding adjustment to the melody.
But, no, the harmony capitulates yet again and moves to IV as expected. The melody then immediately contradicts it with 4-2 (continuing the
descending broken 1'-6-4-2 of ii7) which are b7 and 5 of the V7 that follows. The last two notes
of the bar are 5-3.
And the harmony votes its agreement by moving to vi, which is then followed up
melodically by 1 (3 of vi) in a rare case of the harmony getting a word in before the melody.
So now we can appreciate the melodic motion of the original. In bars 5
through 7, the melody notes have gone from C to F-D-Bb-G- (a descending broken
ii7, which develops as the harmony moves through ii and V7) then C-A-F
(a descending broken I over vi, or arguably most of a descending vi7
over vi). These descending broken chords signpost the way back to the tonic,
like landing-lights, or blood returning to the heart. The rest of the song is more or less a repeat of bars 5 and 6. Because the harmony
at the end of the bar is I and not I7, the 6 is a smidgen (but only a
smidgen) less significant than the one at the end of bar 5.
This is essentially bar 6 except it ends not with 5-3 but with 7-1'. The 7 was already the 3 of V7 but the 1' is in disagreement with the sustained harmony.
And the harmony catches up for the last time and its I agrees with the sustained 1.
