William Nicholson had more than a passing interest in the Oscars this year  his most recent screenplay is the melodramatic marvel that is Les Misérables. It's no surprise, then, that his novels are regularly described as cinematic. In his new novel Motherland, set during and immediately after the second world war, this means some terrific scenes  his portrayal of the wartime disaster that was the raid on Dieppe is grippingly rendered. But the flip-side is that there are regular pages of unmediated dialogue, as if Nicholson has tired of having to do something as boring as describing deep emotion.

And emotional depth is important here because for all its historical nods to the war, the partition of India and, er, the banana business, Motherland is a love story in which troubled commando Ed and arty officer Larry both fall in love with the same alluring girl, Kitty. Ed is the haunted-but-charismatic one who wins her heart early on, but panicked by what he sees at Dieppe and the notion of heroism, his darker side begins to impact on their relationship. Meanwhile, Larry struggles to be good and fair to his friend on the sidelines. "Something happened to all of us [at Dieppe]," he says portentously. "It was like the end of the world."

It's not the only time Motherland tramples into cliches of frightfully stilted characters talking in incredibly broad brush strokes about love and Important Moments in History. Nicholson's last book, The Golden Hour, felt like an attempt at thoughtful, commercial modern fiction, but Motherland is just a bit obvious. It'll probably make for a smash-hit film.

It comes as no surprise, reading the opening paragraphs of Motherland, to discover that William Nicholson is an award-winning screenwriter, with movies such as Shadowlands and Gladiator to his credit. The novel introduces its characters with filmic economy: Alice Dickinson, a young English woman, is on her way to France to visit her grandmother who, until 10 days before, knew nothing of her existence. She has also just ended an unsatisfactory love affair. Herself an accidental conception, Alice comes, her father tells her, "from a long line of mistakes". It is left to her grandmother to tell her the family's "one true love story".

As the story winds back to the summer of 1942, the suspicion that this is a film script pretending to be a novel lingers. The jolly ATS girls, the chinless aristocrat whose house has been commandeered by the army, the stiff-backed brigadier on a Sussex cliff top that commands a view "almost parodic in its Englishness", the clipped dialogue about the upcoming "big show": all combine to conjure the comforting familiarity of a Sunday night TV drama.

But as the story gathers momentum, it is plain that Motherland is a great deal more than a movie-in-waiting. Stationed in Sussex are two friends, Ed, a Royal Marine commando, and Larry, a liaison officer with Combined Operations. Both fall in love with the beautiful Kitty but it is charismatic Ed, with his shadow of sadness, who steals Kitty's heart. After a whirlwind courtship they marry but, almost as soon as the wedding is over, Ed is recalled from his honeymoon to take part in Operation Jubilee, an attack on occupied Dieppe and the largest combined operation of the war so far. Unable to stand on the margins, Larry volunteers to go too.

Dieppe, of course, was a bloodbath. Of the 6,000 soldiers involved, nearly three-quarters were killed, injured or captured by the enemy. Larry is wounded but manages to get out. Ed is taken prisoner. It will be three years before he returns home to the woman he has known barely a month.

Nicholson captures with economy the noise and chaos, the sickening terror of battle, but it is the long shadow of the catastrophe, its terrible psychological legacy, that interests him more. In war, under the influence of what Ed calls the "intoxication of self-sacrifice", life-changing decisions are quickly made. It is when war is over that there is the time and the necessity to consider what truly makes a life well lived.

It is Larry's story that shapes the novel and it's no coincidence that one of Kitty and Larry's recurring conversations is about "characters in books, and how hard it is to make good people interesting". Larry is that rare creature, a good man who is neither smug nor a drip. Clever, kind, his own worst critic, he wrestles with his conscience and his desires; a dreamer who doubts the validity of his own dreams. Though he longs to be a painter, he fears always that he is not good enough: "it's the gap between what he felt as he painted them and what he feels seeing them now that is so unbearable". Ed, on the other hand, is blithe  "No fear, no shame, no hesitation. Live your life like an arrow in flight," he tells Kitty  but battles with the demon of depression.

Nicholson is a deceptively simple stylist but Motherland is a profound and moving novel; a tender and compassionate meditation on love and God and duty and how to be good. "How little we are," Larry says bleakly, "how ridiculous, how lost, in the eye of eternity", and, even as we know it to be true, we cannot keep ourselves from caring deeply about the fate of these decent, flawed people. As the story moves from Cathar France to post-colonial Jamaica to India during Partition, it is impossible to forget mankind's history of cruelty and bloodshed, and yet there is hope, because there is always, in Nicholson and his characters, however desperate and lost, the small fierce spark of humanity. His great triumph is in creating a love triangle in which we, as much as the characters themselves, can see no resolution, because it is unbearable to imagine any of them hurt.

 Clare Clark's Beautiful Lies is published by Harvill Secker.