βehemoth opens five years after rifter Lenie Clarke, in an apocalyptic act of vengeance, seeded the deadly microbe βehemoth across a North America already reeling from out-of-control disease and environmental collapse. No living thing has any defense against βehemoth, and the entire biosphere is dying. Elsewhere in the world, governments frantically try to stave off contamination, and wage a losing battle against the destructive cult of the Meltdown Madonna, a dark mythos spawned by Lenie’s Typhoid Mary-like odyssey. CSIRA, the rapid-response agency whose task it was to confront and contain the endlessly multiplying crises of a pre-βehemoth world, is still active, though it can really only delay the inevitable. Its last outpost in N’Am is manned by Achilles Desjardins: best of the ‘lawbreakers, heroic fighter of a rearguard action on a doomed continent--and also, unknown to his superiors, a monster, a sexual sadist and psychopath, whose involuntary release from the neurochemical restraints that once prevented him from acting on his desires has allowed him to indulge them to the full.

Deep beneath the surface of the Atlantic Ocean, a secret underwater habitat called Atlantis provides refuge for a cadre of the powerful corporate executives whose greed, as much as Lenie’s anger, brought about the destruction of the world. To Atlantis also have come the last of the rifters, their engineered bodies perfectly designed for life in this most hostile of environments--including Lenie, now tormented by remorse, and Ken Lubin, the assassin who hunted her across N’Am and has become her friend (if a relationship between two such damaged souls can be called friendship). The corpses fear the rifters, who are a constant reminder of their vulnerability, and the rifters hate the corpses, who created and exploited them--two rival tribes, bound in uneasy symbiosis by their mutual confinement to the ocean floor.

This brittle peace is about to be shattered. Atlantis’s builders sited it in an area thought to be clean of βehemoth contamination, but a rifter’s near-fatal encounter with a freakishly mutated leviathan reveals the microbe’s presence--a new strain, even more deadly than the original, which they christen β-max. The rifters’ paranoia flares--are the corpses trying to get rid of them once and for all? Amid the rising tension, Lenie and Ken make an even more terrible discovery: β-max may indeed have been deliberately seeded, but not by the corpses. Someone back in N’Am has discovered Atlantis, and is trying to destroy it. For the first time in five years, Lenie and Ken must leave the ocean for the dying mainland, in a race to find their enemy before their enemy can eliminate them.

Like its predecessors, βehemoth is a taut thriller fueled by cutting-edge scientific speculation, whose fast-moving plot doesn’t neglect the subtleties of character. Watts presents a world that is recognizably our own, yet as alien as a distant planet: the microbe-ravaged mainland, where human beings have withdrawn into shielded towns and cities whose protection is only a temporary stopgap (Ken and Lenie’s approach to Achilles’s fortresslike headquarters, looming like the tower of Isengard amid a trashed urban landscape, is especially memorable), and an intensely atmospheric evocation of the claustrophic ocean depths, where the rifters, living out their aimless post-apocalypse existence, are ever-so-slowly devolving toward the level of the ocean creatures whose harsh environment they’ve been engineered to share. It’s a profoundly dystopian vision, plumbing the blackest depths of the human psyche (especially the gruesome segments from Achilles Desjardins’s viewpoint) and the ultimate extremities of environmental disaster, with little room for hope on either front.

Watts continues to explore themes raised in previous volumes, especially the scientific hubris that, as much as Lenie’s deadly odyssey, is responsible for the world’s destruction. In βehemoth, we learn precisely how this arrogance, allied with coprorate greed, planted the seeds of disaster. There’s also the Frankenstein-like relationship between the rifters, deeply damaged individuals injured further by those who engineered them beyond humanity, and the corpses, the rifters' abusers and creators. Between Lenie and Pat Rowan, the corpses’ leader, this tension has been transmuted into a fragile bond, with each recognizing the other’s terrible burden of guilt. For the rest, the fear and hatred are only dormant, ready to wake at the smallest of misunderstandings. The process by which this occurs, and the violent, inevitable results, spur the main action of the novel’s first half.

Questions of guilt and conscience dominate the second half, when Ken and Lenie return to dry land. Such questions have run throughout the series, with its cynical portrayal of the tyranny of the greater good (which can sometimes be achieved only through the commission of atrocity), and its acute examination of the meaning of moral responsibility, when conscience is a product of altered brain chemistry. Here they’re presented through a trinity of characters: Achille Desjardins, who didn’t choose the neurochemical freedom that released his psychopathy, and therefore believes he isn’t morally accountable for his hideous behavior; Ken Lubin, a sociopath similarly released, who, in full awareness that he owns neither conscience nor the ability for guilt, chooses to “play by the rules,” behaving as if he possessed both; and Lenie Clarke, whose conscience is fully functional and who is profoundly driven by remorse and the desire to atone. The actions and interactions of these three compose a complex morality play--and also demonstrate the impossibility of reducing human behavior to its chemical components, even though all human behavior is based in brain chemistry. “Some readers may wonder if I have trouble distinguishing between personality and neurochemistry,” Watts comments a little ruefully in the Notes and References section at the end of the book. I think he makes the distinction very clear.

This is the most memorable SF I’ve read so far this year-- absorbing, thought-provoking, and above all intelligent. It’s a terrific conclusion to a notable series.