David Mitchell may first have come to the widespread attention of genre readers with 2004's Cloud Atlas, a sprawling, nested novel of competing and complementary narratives, the chief achievement of which was structural; but from his very first novel, 1999's Ghostwritten, Mitchell has in fact been up to something very familiar to habitués of SFF: he has been building a megatext, a single, expansive novelistic history of the future. That is, he has been invisibly building a super-structure above the already intricate constructions of his novels.

The question one must ask of this endeavour is complex in its simplicity: why?

The narratives of Cloud Atlas offer a version of Mitchell's apparent approach in microcosm: they share motifs and names, themes and spins on events. At first, they seem disparate and unconnected, featuring a near-future clone-slave in one strand and an Edwardian composer in another; but as one reads, it is possible to discern—perhaps to imagine—the shapes of tendons and connections. In the manner that a reader of Cloud Atlas might have proceeded to Mitchell's autobiographical Black Swan Green (2006) and found its depiction of 1980s Worcestershire surely irrelevantly quotidian, an inspection of 2010's The Thousand Autumns of Jacob de Zoet would nag that, perhaps, there was something more to Dr Marinus than met the eye; it was only in 2014's The Bone Clocks, however, that Mitchell revealed the earlier novel to be very much part of his rather weirder wider project and Marinus indeed to be an immortal. In turn, the novel included a minor character from Black Swan Green and ended with scenes that might be read to begin to lead towards the futures depicted in Cloud Atlas; as in that single novel, then, reading the Mitchellian megatext in toto is a contingent experience, a slow-boil epiphany.

But what, precisely, is Mitchell revealing?

It might at first glance appear odd to hang so deep a probing of one of Britain's most prominent writers of literary fantastica on a jeu d'esprit; but Slade House, which proceeded out of a short story first published in the form of 280 tweets, is in its slightness almost refined Mitchell, the megatext distilled. Brevity is in a curious sense both a quality Mitchell struggles to master and his characteristic approach: his novels are long and full of detail, but also bitty and episodic. Slade House—both slim and, of course, segmented into separate stories revolving around a central theme—feels, like Cloud Atlas before it yet more so, like a scale model of the whole.

This is particularly the case in its fourth chapter, "You Dark Horse, You," but let's pretend Mitchell adheres to linearity and start at the beginning. The titular abode is a grand townhouse hidden down a back street—Slade Alley, natch—of an unnamed university town an hour from London Paddington. In it live Norah and Jonah, twins with a very deep connection who live the high life on the basis of an attenuated connection to a faded family of the landed gentry. So far, so classic-country-house. The inevitable Mitchellian twist, of course, is that Norah and Jonah have been inspired to vampiric immortality by knowledge of the Shaded Way, the evil circle of immortals which constituted the improbably irredeemable antagonist of The Bone Clocks, and their house is accessible to the outside world only once every nine years via a magic door. In this confrontation between traditional English setting and lurid gothic set-up lies the generic project that powered The Bone Clocks and is apparently of ever more interest to Mitchell—an urge to fuse the literary with the popular, the respectable with the pulpish.

For the grotesque is Slade House's default mode. In order to stay young forever, the twins must feed once every nine years—without fail on the same day of the cycle—on the soul of an innocent they must lure to their mutable townhouse (it exists, of course, not quite on our own plane of reality, and, like the TARDIS, can reface and reshape itself at will). The chapters of the novel, then, follow in first-person singular the story of an innocent each, taking them with increasingly dreadful inevitability from obliviousness to oblivion. Each time, the twins position their captive before a mirror, and we watch each of them watch themselves simply disappear in its surface, their soul sucked in a mist from their physical form in order to feed the twins' ever-lasting, ever-youthful, life.

The first such victim is the one who appeared in that Twitter drabble: Nathan, the son of a pushy-but-failed musician medicating her post-divorce stupor with Valium. He finds himself at Slade House in 1979, his mother lured there by the promise of an unlikely audience with Yehudi Menuhin. Nathan spends his time playing with Jonah, a boy of his own age who apparently lives in the house, asking his new friend if he ever thinks he "might be a different species of human, knitted out of raw DNA in a laboratory like in The Island of Doctor Moreau" (p. 20). The irony here, of course, is that Nathan is one of the Engifted, a rare sub-stratum of humanity possessed of psychic abilities—and precisely the individuals the Grayer twins must feed off in order to survive.

Mitchell's name-dropping of that Wells novel tips the wink as to how dark he intends his own book to be: Nathan is led by the nose up a flight of stairs, along which portraits of people without eyes (including one of him) are hung, before waking up in the spare bedroom of his father's home in Rhodesia, apparently in the wake of a nightmare—before in turn waking again in Slade House, this time paralysed and about to be soul-eaten. It is a disorienting turn of events which keeps the reader guessing and is recalled in the second chapter—set in 1988 and following a hapless policeman, Gordon Edmonds, investigating a tip-off about the disappeared mother and son of 1979—in the form of an expertly-conjured dread: "This happens in Pornland," Edmonds muses during a post-coital moment in the mistress's bedchamber at Slade House, "but here in the real world, women like Chloe simply don't shag men on a second encounter. Do they?" (p. 66). We know, of course, that Gordon is not in the real world, and he, too, is promptly walked past the portraits—now including a sightless one of him—and sickeningly dispatched.

This procession of despair might reach its bleakest moment in the third chapter, which introduces Sally Timms, an unhappy new undergraduate with body image issues (the chapter is entitled "Oink Oink" and Sally wears a pig mask to a party); Mitchell builds Sally's character carefully, introducing us to her hopes, dreams, insecurities, and troubled past in just enough detail that her inevitable death stings all the greater. Gordon Edmonds is toyed with at the end of his life, forced to recite a Lord's Prayer which he can only dimly remember and will have no effect anyway, but in the process is given a brief hope which, when broken, lends his soul extra flavour; Sally simply proceeds through the awkward, self-loathing sort of house party many young people might attend—and then is summarily ripped from her youth with the belated realisation that, "My soul's the most beautiful thing I've ever seen." (p. 138) At times, Slade House is wicked in its expert horror.

Indeed, the Grayers are pretty impressed with their work on Sally, too: "If the Academy awarded Oscars for Best Orison," says Jonah to Norah, referring to the grand illusions they put on every nine years at Slade, "you'd be a shoe-in. Truly, it was a masterpiece. Cubist, postmodern—superlatives fail me" (p. 136). Again, the novel is self-referential: Mitchell, too, is engaged in a mash-up of styles and approaches, fusing the haunted house with the crime procedural, the YA slasher with cosmic horror. He did much the same in The Bone Clocks, albeit across a much larger canvas and in a more segmented way: in that novel, there was a Fantasy Chapter and a Literary Farce chapter, and so on; in the grand guignol of Slade House, genres abut each other in the same sentence. This makes everything seem much smoother and rather more elegant, but some of Mitchell's weaknesses recur. In particular, each chapter ends with a crushing bout of exposition, in which the Grayers talk like the worst comic book villains, spilling their plans and motivations to their helpless victims in the moments before their deaths (and ignoring the fact that only the sacrifices' organic matter disappears, leaving their psychic presence and physical remains—such as pig masks, lockets, and hairpins—behind, to relay clues and cause later havoc). "Why this illuminating lecture now, Sister?" groans Jonah at one point, echoing the reader's thoughts more or less precisely (p. 79).

This is a feature—and a bug—which readers of The Bone Clocks will recognise. In truth, Mitchell's megatext seems, too, to be a retcon—at this year's Edinburgh international book festival, the author revealed that it first occurred to him that his novels took place in a single secondary world during a relatively recent conversation with his editor—and it's not entirely clear that his fiction yet knows quite how to fit into this new scheme, or what to do with it once it figures that out. Paul Kincaid has argued convincingly that we should best read Mitchell's work as pattern- rather than story-making, and that in this light the unwieldy verbiage, and flabby structures, of The Bone Clocks are a function of its simply trying to draw together too many of Mitchell's themes, motifs, and characters. In Slade House, however—a much slimmer and more focused work—Mitchell still struggles to layer his world-building organically. Rather, he lays down the cement of his world-bricks with a honking great trowel.

This is a pity, because as we've come to expect from Mitchell there are moments of simply wonderful writing—in the Sally sections, yes, but right through to the final page, on which a "magpie lands on a garden shed with a crawk and a tinny thump" (p. 233). Central to Mitchell's project is aligning the literary with the generic, and few would argue that he has not been an important figure in contributing to the increasing fluidity of this border over the last ten years. His novels are self-aware in a fashion which sometimes strikes one as having their cake and also scoffing it, but which is also usually effective: treated to a long backstory about the Grayers and their immortal travels that turns out to be entirely true, a journalist scoffs, "This is all sounding a bit Da Vinci Code for me" (p. 151); later, trapped in an Orison, she asks herself, "Really, which is likelier? The laws of physics breaking down or a stressed-out journalist breaking down?" (p. 184). It is the purpose of Mitchell's fiction that, where in literary novels one of these options is always true and in SFF the other, things can in his own megatext go either way. This is an enjoyable and exciting world to read, and it is always written with brio.

Indeed, with characteristic confidence Mitchell makes all those Bond-villain soliloquies part of the novel's denouement—the cackling, over-confident villain "an aptly named Jonah" (p. 224)—and re-introduces Marinus, the endlessly-reincarnated Horologist from Thousand Autumns and The Bone Clocks. Marinus is essentially Mitchell's super-hero, and her arrival here somewhat leaches tension from the climax: given this is a minor entry in a grand megatext, from the moment Marinus's famous name first appears, the Grayers are doomed. Marinus plays along with the Orison for some time before intoning at the most dramatic moment, "Why in the eleven thousand and eleven names of God would I oblige two parasitic soul-slayers by imbibing their poison?" (p. 219). And with one bound, she is free! Perhaps not quite anything can happen in a Mitchell novel; shackled inside the megatext, a novel's events become reliant on where it sits in the greater whole.

Mitchell is, then, still struggling to square his own circles. Generic markers are tricky things to break down, and the fact that Mitchell has made them even as fluid as he has is a marvel. Slade House, indeed, is an eloquent adventure story about grief and loss, and at times I wished Mitchell could content himself with writing great novels of this sort, rather than entries in a grander whole: Marinus is a disruptive presence, and the references to other novels leach away some of the book's exquisite tension. It is in the ever-incomplete nature of a megatext that it is hard to judge until it is complete, and if the whole amplifies and thickens the messages of the parts then Mitchell may well have done the impossible by the close of his career. As it is, however, the reader and writer are still feeling their way, wondering—sometimes to distraction—what the wider significance of it all might be. "What's a metalife without a mission?" asks Dr Marinus in the course of Slade House. "It's mere . . . feeding" (p. 230). Given how well Mitchell writes, one hopes that we and he both can resist the urge to ask that question in future, and enjoy the individual novels for what they are. The megatext might turn out to be a lodestar; but on the evidence of Slade House it may just as easily be unnecessary ballast.

Dan Hartland's reviews have appeared for some years at Strange Horizons, as well as in publications such as Vector, Foundation, and the Los Angeles Review of Books. He blogs at thestoryandthetruth.wordpress.com.