China Mieville's "The Scar" is set in the same universe as "Perdido Street Station", but is not a sequel. It follows a very strange journey of a woman named Bellis, as she flees New Crobuzon by boat in the post-Perdido-Street-Station fallout. Her goal is to emigrate to a faraway corner of Bas Lag, but her trip takes a wrong -- or rather, strange -- turn when the ship is hijacked by, and incorporated into, a floating conglomerate of ships called Armada. Armada is, loosely speaking, a pirate society. It belongs to no country and obeys no laws except their own. It is governed by several mysterious figures. Soon it becomes apparent that Armada targeted this particular ship for deeper reasons than just to enslave its passengers and take their stuff. The leaders of Armada seem to have their own goals they don't share with the common citizens. Since their fate is at the mercy of the rulers, the captives have a strong interest in finding out what's in store for Armada. Plotting and scheming ensues, punctuated by encounters with improbable sea creatures, and carriers of ancient, secret knowledge.
Plot tension not sustained
The premise of the novel is interesting, yet this book suffers from the same affliction as "Perdido Street Station", only to a much larger extent. The plot does not progress seamlessly. It lurches along in fits and starts. Individual characters' mysteries, along with hints that something Big is about to happen to Armada, may be enough to sustain the reader's interest, but not very consistently. This book did not keep me glued to it. The plot tension rises and lets up, but not in any meaningful rhythm. Just when you hope the plot is thickening, it suddenly plateaus and diffuses, and we're back to following Bellis's mundane daily activities. Yes, life in a floating anarchist state apparently can get very mundane, even indistinguishable from a middle class urban dweller's existence. This could be partly because Armada put Bellis to work as a librarian. We get to follow her along as she goes to her boring job, sulks, pines for New Crobuzon, and makes painful attempts to connect with people who are positively bad for her. Well, she meets a few good folks too, but somehow her interactions with them aren't nearly as interesting as with the bad guys. :-). They bring tension to the plot, rekindling our interest just as we were about to forget why we were reading this book.
The ending does indeed justify the expectation that something Big is bound to happen, but it does not directly follow from the events in the plot. I got a feeling that the key plot threads were tied up retroactively, rather than being thought out from the beginning. The ending is not really predictable, but neither is it convincing. It feels a bit arbitrary.
Interesting characters compensate for plot deficiencies
That said, "Scar" has a lot to recommend it. I found the characters more interesting than in "Perdido Street Station", perhaps because I could identify with the brooding, misanthropic, individualistic, suspicious Bellis. She despises Armada's attempts to integrate her. Even when she relents, she continues to secretly weave her own plans. But as much as she resists it, her perspective of both New Crobuzon and of Armada starts to change when she meets people who were oppressed in New Crobuzon and found freedom, paradoxically, as citizens of Armada. The ambiguity of her situation and the promise of freedom in captivity is one aspect of the story that makes it interesting to see how it plays out. In the process she makes some critical mistakes and suffers harshly for doing what she thought was right.
I have to say I was surprised to learn the point China Mieville tried to make with Bellis's fate. In one interview he said Bellis's punishment for doing what she thought was right goes to show that a person's fantasy of singlehandedly saving the world is futile, and change can only be brought about by organized mass resistance. If this was the moral of Bellis story, I don't think it came through very well. Bellis did not act out of exaggerated sense of her own importance. She honestly thought she had the right information and simply could not stand by and allow a disaster to happen. There was no way for her to know the information she was acting on was wrong.
Still, the message that only the masses can effect change came through loud and clear in the end for different reasons. It just wasn't the reason for Bellis' failure.
Several secondary characters also make this book worth reading. As is often the case in fantasy, the mysterious bad guys are more interesting than the goody-goodies. :-) There are hints at a budding romance between characters who would make a rather unlikely couple, and I was very interested to see how it was going to turn out.
A good thing about "The Scar" is also that it is not overloaded with monsters. Sure, there is a healthy dose of them, and some of encounters with them are more or less gratuitous (the episode with blood-sucking anophelii seemed like a detour from the main story) but they are less chaotic and more integrated into the plot than in "Perdido Street Station". Since most of the book takes place in the sea, all the creatures encountered along the way are part of the ocean ecology. So the worldbuilding in this novel seems more orderly and therefore more convincing than "Perdido Street Station".