A lot of small, unusual, interrelated ideas packed into this song's two minutes and nine seconds. It starts with a two-bar riff on the tonic and major third of F, but then tonicizes the third (A) for the first verse. That major third (chromatic mediant) relationship is then heard again in the first chord progression of the verse: A major to C# major. These chords are repeated once and then shift down a whole tone (G major to B minor). The shift is momentary, however, and then it's back to A major and C# major, both functioning as the fluctuating tonic.
The verse is followed by another guitar/bass riff, starting with the same descending third heard in the opening riff but transposed up a major third (C# down to A). The riff then involves a higher F in a descending line moving down to the low F, showing us now that these notes are actually spelling out an augmented chord (F, A, C#, F). The last two bars of this three-bar riff are actually the same two bars that were heard at the beginning of the song, thus setting up the second verse in the same manner as the first.
The second verse has a different ending than the first, involving a little more dramatic diatonicism when the dominant chord in A is reached. Instead of resolving to A, however, the dominant E moves down to C# major, keeping up that same A/C# tonic fluctuation.
C# then serves as the key for the singsong bridge, played at a different tempo. After the bridge, the guitar and bass once again make their way back to that same two-bar riff on the tonic and third of F, making clever use of the major third interval (a big element in this song) along the way. What follows, finally, is an abbreviated third verse with two lines, this time just moving up from A to B, then A to B to C# (all major chords). After the C# chord, the guitar and bass play the same return to the initial riff as heard after the first verse, but this time using the F as a weird dominant prep chord that leads to G#, the dominant of the final tonic resolution in C#.*
* C# is heard in the bass, anyway, though Mayo Thompson does not play a straight C# major triad on the guitar on the last beat.