One of the weirdest songs by Genesis and one of the most fascinating ones too, IMHO. Being a "big classic" of prog rock, I won't produce here a detailed description (well, I couldn't anyway), but I can't avoid some personal observations. The entire song is full of pure prog energy, packed of a special kind of sparkling, juvenile, dynamic fury. The sci-fi theme (a rather scary strory about a killer plant, reminiscent I think of John Wyndh'am triffids) creates a claustrophobic atmosphere, increased by a series of crescendos and quick tempos.
The only break in such a rush is when the electric piano comes in or when it comes to a musical depiction of the most British country gentleman, but even then you'll feel a sharp tension, an impending-danger mood that rules the music. Useless to say, the themes, the solos and the interplays are very well found, maybe a little rough, but that's exactly what you need to describe a deadly struggle. I heard many people say that a song like this explains why Genesis are a world apart in the progressive universe. I bet they're right.