Submergence, By J M Ledgard

James More, a paratrooper turned intelligence officer, posing as a water engineer, is held hostage by jihadi fighters on the Somalian coast. Not necessarily swallowing his cover story, they call him Mr Water. During his incarceration, he remembers a woman he met at a hotel in France, also on the coast, Danielle (Danny) Flinders.

Danny is a biomathematician specialising in the study of microbial life in the Hadal zone – deep-sea trenches, more than 6,000 metres down, where chemicals alone can sustain life. Danny believes that an understanding of such life is necessary for human survival. She is a strong, independent woman who will normally take a random crew member into her bunk for the duration of a research trip. She and James fall in love in the hotel, which, not for nothing, is called the Atlantic.

On the coast of another ocean, James is frogmarched into the surf and subjected to a mock execution. As a child, James lived by the North Sea and played a game in which he had to walk across a tidal river, at one point becoming submerged: "You had to hold your nerve." The fighters take him to an Iraqi doctor and then on a voyage by dhow.

This novel is a deceptive scrapbook of detail and abstraction, of fictional narrative and encyclopaedic interludes. A heavier hand might render these didactic but Ledgard writes beautifully and in his own style, whether describing Masai initiation rites, huge-eyed deep-sea creatures or the delicate calibrations by which you might realise you are falling in love.

Chronology is fluid – very occasionally, at the expense of story. There are jaw-dropping suggestions – that the paradisical backdrops to suicide bombers' videos might be influenced by exposure to Disney films, in particular Bambi – and a harrowing episode describing the stoning of a young girl, a tightly controlled piece of writing of awesome power.

Themes slide across each other like thermal layers in the deep ocean. Danny searches for answers in the Greenland Sea while James retreats into utopian fantasies. In a profound meditation on cruelty, pity, belief, art, science, hope, love and mortality, the novel's truths settle in your consciousness, perhaps never to be forgotten.

Nicholas Royle edited 'The Best British Short Stories 2011' (Salt)