That's because there is no plot. I finished it today. It's pretty typical Gibson--clever ideas, stunning visual description, and no cogent story to speak of. I found it even more aimless than his other books, however, so I'm inclined to pin a lot of the blame on Bruce Sterling, who I have never read before. In essence, though, "The Difference Engine" is a casual assemblage of ideas in which the only conclusion or climax comes from a subplot that is never developed "on screen."
More than the plot, though, the thing that really bothered me is that they seemed to be on autopilot for most of the book. They had a chance to do something stunning with setting and speculation--to show what might happen in a world where computer technology arrived so early--but apparently it's just the same things that happen when computer technology arrives on time, just dressed in different clothes. While that may be accurate, I found it disappointing; I was expecting something more, I suppose, than just another cautionary tale about the loss of identity in a technological society.
In other news, I decided to go with Bleak House instead of Barnaby Rudge. I initially picked up the latter because it dealt with riots and violence, and I wanted to see how such things were handled in that period, but as it turns out The Difference Engine had plenty of riots and violence, so Barnaby Rudge seemed redundant. Bleak House, however, is quite good, and I'm enjoying it quite a bit.
(I enjoyed Difference Engine too, as far as that goes...I just thought it was pulling too many punches.)