FANS OF DAVID MITCHELL know to keep their expectations both high and flexible. The British novelist's previous work includes the Booker-nominated Cloud Atlas, which marries six interlocking stories and genres into one ingenious whole; the charming semi-autobiographical bildungsroman Black Swan Green; and 2010's foray into historical fiction, The Thousand Autumns of Jacob de Zoet, a meticulously detailed and relatively straightforward account of a Dutch shipping outfit in the 19th century.

Few contemporary authors switch modes so elegantly or so compulsively as Mitchell—so it shouldn't surprise anyone who's been paying attention that he mixes things up once again with The Bone Clocks, his first stab at what can almost accurately be called "paranormal fantasy."

The Bone Clocks introduces Holly Sykes, aged 15 when the novel opens, a British schoolgirl who's both newly heartbroken and just a little bit psychic. As a young child she heard voices—"the Radio People," she called them—and as a lovesick teen, she proves a magnet for forces far beyond her understanding. There's a battle playing out, see, between two bands of mysterious immortals: The Horologists, good guys who are reincarnated in a new body every time they die; and the Anchorites, bad guys whose eternal youth relies on a regularly replenished supply of (violently murdered) innocent souls. During an adolescent runaway attempt, young Holly forms an unwitting alliance with the good guys—an alliance that will shape the course of her life.

Earlier this year, JK Rowling released a story touching on the post-Hogwarts life of Harry Potter. In the same vein as the final book's epilogue, it forcefully reminded readers that Harry et al. grew up, had families, went on to have meaningful lives. All the Harry-as-grownup stuff couldn't help but feel a bit perfunctory, though—everyone knows the really important parts of Harry's life happened at Hogwarts, years 1-7.

By contrast—not just with Harry, but also Lyra, and Katniss, and so many other young heroes of fiction whose defining moments come in adolescence—Mitchell refuses to let Holly's story end at age 15. Instead, he does something really bold: He lets her grow up.

Holly is at the center of The Bone Clocks, but the perspective shifts during this surprisingly dense novel to other people who play important roles in her life: There's the childhood friend turned war correspondent; the callow young man—a cameo from Black Swan Green's Hugo Lamb—she takes as a lover one winter in Switzerland; and the once-beloved author whose fortunes have faded, who tries in vain to interest his agent in a new novel that's "one-third fantasy. Half, at most." (Yes, Mitchell, we see you winking.) These outside perspectives let us see Holly through fresh eyes over the years, as she gets older, learns things, has a family, experiences loss. And the adventure that begins when she was 15 doesn't end until she's well past middle age.

The novel feels ponderous and strained during the strictly paranormal sections, when supernatural entities are having massive, good vs. evil battles in the ether—I kept waiting for some Cloud Atlas-style sleight of hand, for it to be revealed that these sections had in fact been authored by some third-party hack. (Alas.) By contrast, the human characters here, with their mundane battles and petty problems and fragile, temporary lives, are as indelible as any Mitchell has created. Because, despite all the good vs. evil battles between psychic immortals, The Bone Clocks is really most concerned with the toll time takes on human bodies: We age. We die. Our faces and bodies tick off the years as they pass—bone clocks, frames marking the passage of time.