A giant squid goes missing from the Natural History Museum, and it turns out to be an actual dead god worshipped by a church who have taken it, through the expedient of seducing a Trekkie into using transporter, AKA die-and-re-live, technology, in exchange for personal use of a working phaser. The squid is kept in the back of a lorry, always on the move so that nobody can find it.
The museum curator responsible for the squid is drawn into the magical underworld, believed to have knowledge of the squid's true potency, but in fact initially oblivious and losing a friend (a clever scene in which `Goss' and `Subby' come through a letter-box having been folded up by an origami expert), and then the story revolves around Billy finding out (as does the reader) about the strange world that exists underneath London, and his dead friend's girlfriend manages by halves to get involved in the sub-culture too—just enough to understand things strangely happening.
It is the elucidation of this strange world that forms half of the book's content, and is uncovered through Billy's adventures and his discovery of the past lives of the protagonists, predominantly a man imprisoned as a tattoo on another man's back, and his former accomplice who is left to live a life simply as ink—his desire to get his hands on the squid is simply that he can gain huge power in the magical underworld by mixing with the god's squid-ink. China Mieville really stretches the imagination beyond limits; he manages to pull off the ink-person with the help of an ally who actually moves him (writes) on pieces of paper on which he can form words, fly(!), and experience the environment himself, but the idea that they constantly flush the ink back off the paper and into a large jar without losing anything, over several years, is pushing things. A much bigger push is the idea that the tattoo, once supposedly the most powerful thing in the underworld (apart from gods), and able to perform biological enhancements to specification, has as bodyguards men whose heads have been replaced by giant fists… and then there are the memory-angels which take the form of large specimen jars with a skull at the top and which somehow move around making glass scraping sounds.
An interesting character is Wati, a soul living in the aether and which observes the world through the eyes of statues, mannequins and dolls, with the potential to be omnipresent and thus smother the story. Mieville avoids this by keeping Wati occupied, out of the narrative, with striking animals who threaten to expose themselves to the general public by, for example, pigeons walking round in circles outside the British Library in one amusing scene.
The attempted climax of the book is first a failed double-Armageddon brought on by two factions (the squid church and the ink-villains) simultaneously failing to end the world by burning it, and then a house full of water is penetrated causing a minor flood and letting the detectives in to finally re-capture the squid. (The Embassy of the Sea is one of the outlandish things that Mieville actually manages to pull off; the sea is another, actually very powerful, god).
The good points are that the build-up of foreboding, of menace in meeting some of the protagonists is very well done, and does lead to a sense of real horror even if such never really materializes. The subtle development of the background history to the story is also very clever and skillfully done—at least for half the plot.
The main problem I've had with this book is the juxtaposition of the Ian Rankin-style casual intercourse between police officers and the surreal world of magical London, impressed upon, but living beneath the real London. In short, the dialogue is not well written, believable, and does not sit with the high-powered super-world that the officers are supposed to be investigating.
I've not enjoyed reading this book so much as I would have hoped. Mainly it is the constant development of ideas; there is no point at which you feel the story is constrained by the universe. The worst part was the dismissal of the Goss and Subby characters, who are mysteriously all-powerful through the first three-quarters of the novel, and then it transpires that a stab at one kills the other.
On the whole, it is a book composed of parts ludicrous, parts over-convenient, parts too mundane to fit, parts genuine if unrealized horror, and parts of colourful story. The whole does not fit together well. On top of all that, the universe is totally alien but grafted onto the real London town; real people simply do not see the magical world that co-exists there. This is neither removed enough from reality to allow super-physics, yet not attached enough to a tangible universe for the story not to be unpredictable. But Mieville does manage to conjure some substance with clever prose and the workings of back-story, and does succeed in bringing outlandish ideas to light.