It’s impossible to talk about Peter Watts’s most recent novel, Echopraxia, without discussing his previous novel, Blindsight. The relationship between the two books is crucial to any understanding of Echopraxia.

Blindsight, which appeared in 2006 and went on to collect a flurry of award nominations and no shortage of critical praise, was a significant, even singular work: a sophisticated and challenging thought experiment that meditated on the nature of sentience, consciousness and intelligence. It did so by means of a first-contact story with an impossibly inscrutable alien and an encounter team made up of people that can only be described as on the outskirts of human intelligence: a synthesist with half a brain, a person who’s deliberately subdivided her brain into four personalities, a person whose brain is not limited to the meat inside his skull, and a vampire — a member of a resurrected species able to outthink humans by several orders of magnitude. Dark and dense, Blindsight was not an easy book in any sense, but it was profoundly thought-provoking, and one that definitively made Watts’s reputation as a serious practitioner of neuroscience-based science fiction.

Echopraxia, coming eight years after the publication of Blindsight, is something more than a sequel — or as Watts has called it, a “sidequel,” since its events are roughly contemporaneous with those in Blindsight. It consciously exists in Blindsight’s shadow, much as Ender’s Shadow does with Ender’s Game, or Rosencrantz and Guildenstern Are Dead does with Hamlet. But it’s not just a “home front” narrative, describing what’s taking place on Earth while the events of Blindsight unfold; Echopraxia shares the same themes and scientific concerns (neuroscience, evolution and philosophy) as its predecessor.

It’s been argued that you can read Echopraxia without having read Blindsight, but I’m not sure I agree. While the events of Blindsight took place far from Earth, and the outcome is not known to the players in Echopraxia, they are very much the elephant in the room. Echopraxia is the literary equivalent of a facultative hemiparasite. While it can survive on its own, it derives many of its nutrients from its host.

The world of Echopraxia is one of where most of humanity is either upgraded or uploaded. The protagonist, Daniel Brüks, is a mundane outlier: a baseline human whose mind has not been tweaked, modified or augmented, and who must take drugs to keep up with everyone else. A scientist who has exiled himself to the desert for his past sins, Brüks dissects undead garter snakes and other desert fauna to map the spread of zombie genetics. His isolation is interrupted when a zombie army, controlled by an unleashed vampire, Valerie, attacks the nearby monastery of Bicamerals, who comprise a hyper-intelligent hive mind. But, facing an even greater threat, Valerie and the Bicamerals arrive at a truce, and flee the planet altogether, taking Brüks with them. Also along for the ride is Colonel James Moore (whom we first saw in “The Colonel”), who is our main connection to Blindsight. It emerges that Moore is the father of Siri Keeton, the earlier book’s hemispherectomied protagonist, and is obsessed with getting any news he can from Keeton’s mission.

There’s an awful lot going on, and an awful lot to take in (“The Colonel” is actually a helpful introduction to the world of Echopraxia). And for both Brüks and the reader, it’s not always clear what’s going on. Watts appears to be following Tiptree’s dictum to “start from the end and preferably 5,000 feet underground on a dark day and then don’t tell them.” For nearly the entire novel Brüks appears to be little more than a superfluous observer who’s just along for the ride; I’m reminded of Roger Thornhill in North by Northwest, swept into an adventure he had nothing to do with, entirely by accident. As a baseline, Brüks is the least intelligent being in view, and possibly the least interesting: a parasite — or, as one character calls him, and not at all pejoratively, a “roach,” because baseline humans are less failure-prone than their heavily modded but low-functioning brethren. He does not appear to be necessary to the mission — or the story — at all, except to provide the reader with a viewpoint that is not profoundly unrelatable. But here Watts is misdirecting us: The vampire and the Bicamerals are not escaping; Brüks is not useless, and he’s not there by accident.

In many ways Echopraxia is like its predecessor: It’s a dense, difficult read full of mindblowing ideas, with prose that, typical of Watts, is too good for hard science fiction. But unlike Blindsight, it lacks focus, and in the end the book doesn’t cohere; it reads a bit like a compendium of cutting-edge neurobiological research that has appeared since Blindsight came out, pressed into a novel-shaped final product. The ideas take precedence over the plot, the parts over the whole. Some of this is because, in the absence of a strong plot line, much of Echopraxia exists in relation to Blindsight, without which it kind of flops over. I used a parasitic metaphor earlier, but a better analogy, one also appropriate to Watts’s work, might be hemispheric: Echopraxia is half a brain — Blindsight’s other hemisphere. They can operate independently, and one may be more dominant than the other, but the real action occurs when the two halves function as one.