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Every once in a while, a critic will be mesmerized by a book that stands out from — even wipes the floor with — all other books that have come his way of late. That book is all the more special when it tiptoes unobtrusively onto the scene, devoid of brash fanfare, one of only several new releases from a smallish publishing house. J.M. Ledgard's "Submergence" is that kind of book. For such a slender novel it manages to be grand in scope, rich in content and stunningly executed.

James More is a paratrooper turned British spy, posing as a water engineer in East Africa. He is also a prisoner, held hostage by Al-Qaida jihadist fighters who routinely beat him. So runs one strand. The second concerns half-French, half-Australian Danielle "Danny" Flinders, a biomathematician whose latest project takes her in a submersible thousands of meters below the Greenland Sea to record microbial life. This is the Hadal deep — Hades, hell. "There's a reason heaven is up there," she is told — just as James is being marched by his captors ever nearer toward the target of their suicide mission.

Both characters are plunged into their respective dark abysses. Danny descends and muses on scientific discovery and mankind's prospects of survival. But "Submergence" is more James' story, and to stay afloat throughout his ordeal he ransacks his past, mining anecdotes, plundering memories and replaying conversations that have helped shape him. Snippets from his childhood in the north of England and his intelligence work in Africa (from "Nairobbery" to the badlands of Somalia) are intercut with views on books, art and history. As a descendant of Thomas More, he is given to pondering the validity of utopias, and Ledgard expertly contrasts James' dreams and religious stance with the fanatical visions of martyrdom of the terrorist cell.

The reader is jerked to and fro, from one fragmented thought-filled vignette to another. The occasional disorientation is exhilarating, and James' accrued wisdom reads every time like blended-in knowledge rather than grafted-on data. Some of the flashbacks are memorable for their tenderness, such as James' chance meeting with Danny one Christmas in France at a snowbound hotel on the Atlantic coast, and the impassioned romance that ensues. Others resonate for their horror, most notably a scene James chillingly recalls in which a 14-year-old girl, after being gang-raped, is stoned to death for "adultery." Little wonder James is "abraded by the world."

Scottish-born Ledgard now lives and works in Africa, and his ability to re-create the continent's local color in all its beauty and barbarity is exceptional. Prose merges with poetry; shocks detonate like depth charges, and characters' fates actually matter in "Submergence," an astonishing novel that utterly immerses the reader.

Malcolm Forbes has written for the Times Literary Supplement, the Economist and the Daily Beast. Born in Edinburgh, he lives in Berlin.