Kate Mosse’s vast jigsaw puzzle of a novel, Labyrinth, is a history lesson, a theological thriller, a fantasy adventure involving a trilogy of books within this book, all of it so sprawling, it could have been titled The Lord of the Grail. Part Da Vinci Code, part Tolkien without hobbits, Labyrinth is the British author’s fact-based speculation on nothing less than what makes the world go ’round. (And I’ve just given you your first clue.)

As such, Mosse’s spin on that eternal question pivots around two parallel characters: Dr. Alice Tanner, an academic who, while on a 2005 archaeology dig near Carcassonne, in southern France, stumbles upon two skeletons and a ring; and Alaïs, a young 13th-century woman who stumbles upon the secrets of the Holy Grail. Alternating between the two women, Mosse is able to double the intrigue and to achieve something else: You just know these stories are going to converge, and when they do, it’s the literary version of a thunderclap.

Alaïs belongs to the Christian sect now known as the Cathars, victims of a bloody crusade by the Roman Catholic Church — the church said they were heretics; history says it was a northern-French land grab. Alaïs’ father, embroiled in the struggle, entrusts his daughter with a ”Book of Words” whose value she doesn’t fully comprehend. Similarly, the ring Alice discovers is emblazoned with a circular labyrinth whose ”lines do not lead to the center, as they should.” Mosse keeps her stories neatly paired: Alaïs soon learns she holds one of three books that, together, will either become or lead to the Holy Grail (to clarify that point would spoil the ending), and Alice must figure out why so many others around her, including the mysterious sage Audric Baillard, want the ring. And who are those two skeletons?

Mosse’s prose occasionally suffers from bad editing — when she asks, ”Was Alice made of the same metal?” she probably means mettle — and, frankly, the back-and-forth between centuries, the double and triple crosses, become so confusing that I was relieved when, in the last hundred pages or so, Baillard takes over the narrative to explain to Alice (and us) what the devil (or worse) is going on. It clears things up a bit but also slows the pace to a near halt at a time when most such tales are barreling forward to a derring-do conclusion.

Like many pop phenomena these days, Labyrinth, a best-seller in Europe, has a website (this one overseen by its creator), which serves as a primer to the book’s details and provides numerous suggestions for further research (”If you are interested in the religious debate, try these links…” — followed by a long, highlighted alphabetical list including baptism, dualism, forgiveness, and heresy). The idea is to pull you in with the impressiveness of Mosse’s research, and to gull you into thinking that at least some of the novel’s speculation may be true.

Mosse is no con artist, though, and no Dan Brown rip-off, either, although she doesn’t escape best-seller-prose clichés (”Alice was surprised to realize her heart was beating fast”). Dr. Tanner’s headlong quest for the truth is often exhilarating, and if you can also get hooked on Alaïs’ ancient melodrama (I ultimately couldn’t, though I admired its intricacy), Labyrinth will make for a ripping yarn.

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