The best fiction deals in opposites, explores both sides of a story. Fitzgerald juxtaposes ruin and success. Tolstoy gives us war but, just as memorably, peace. So, too, does acclaimed Scottish author Andrew O'Hagan in his fifth novel, "The Illuminations." In one strand of the narrative we follow Luke, a captain in the British Army, first in battle in Afghanistan and then acclimatizing to a post-military career. Another binary is at work within O'Hagan's other strand, which focuses on Luke's grandmother Anne, a woman in thrall to the early stages of dementia, and who — in trying to remember her life before she forgets it — is "fading away and becoming known at the same time."

O'Hagan opens the novel by introducing Anne at her sheltered-housing complex in a Scottish coastal town. Through conversations with neighbor Maureen and open discussions with a weekly group called the Memory Club, we hear about Anne's remarkable life, from her early years in Toronto and New York to her fateful meeting with Harry, an RAF pilot-turned-photographer.

After spending some time in Anne's company, O'Hagan veers off and brings in her grandson. As Luke waits with his regiment for the next skirmish with "Terry Taliban," he recalls his youth, and how Anne showed him her groundbreaking documentary photographs and instilled in him a love of art and literature. Nostalgic reminiscing is abruptly relegated when his mentor jeopardizes a mission and his unit is ambushed by insurgents.

Uniting these disparate sections is a family tie that turns out to be a strong, inextricable bond. A far more interesting connector, however, is both characters' take on and distortion of truth. Anne's trove of stories, dredged up by a failing and at times hallucinating mind, constitutes a hodgepodge of broken anecdotes and tall tales, which forces the reader to sift, speculate and take with a pinch of salt. Similarly, Luke's version of events in Afghanistan is clouded by confusion ("You couldn't be sure of anything because of the heat and the way reality was bent by the temperature"), and the chaos and disaster from his time there warp his vision of life on the homefront.

"The Illuminations" is a natural extension of O'Hagan's earlier work (aided in part by the reappearance of characters from previous novels) but also an elaborate and ambitious departure from it. The well-researched Afghanistan sequences are gripping. The depiction of Anne moving "in and out of herself" is wrenching. The book's title refers to the lights at an English seaside resort where Anne met Harry, but O'Hagan deftly manipulates the image to apply it to Luke's ordeal. His task — to bring "enlightenment" to Afghanistan; his challenge — to endure the nightly tracer fire in the air, "not a light show but a constellation of death."

O'Hagan, with two Booker Prize-nominations to his name, is a skilled yet criminally undervalued storyteller. With luck, this masterful novel will bring him the wider readership he deserves.

Malcolm Forbes has written for the Times Literary Supplement, the Economist and the Daily Beast. He lives in Edinburgh, Scotland.