By Nick Hornby. Riverhead. 333pp. $24.95 *

At first glance, A Long Way Down represents a considerable departure for Nick Hornby. His books typically revolve around narcissistic men in their thirties, obsessed with sports, music or casual sex, who are disabused of their illusions by patient, intelligent women; his recurrent theme is the movement from adolescence to maturity, from egotism to community. In today's solipsistic age, the stalking ground of that most unlovable of modern phenomena, the "kidult," this is a useful theme to have, and his books, funny and gently wise, have enjoyed great success.

Lately, though, his desire to take on the big questions -- Why are we here? Why should we stick around? -- has led him away from romantic comedy into darker terrain. A Long Way Down opens on the rooftop of a London building on New Year's Eve, where its four narrators -- Martin, a disgraced TV chat show host; JJ, a failed rock musician; Jess, a mercurial teenager; and Maureen, a middle-aged woman with a profoundly disabled son -- meet accidentally just as they are about to kill themselves. Largely out of embarrassment, they abandon their plans and, in the weeks that follow, help one another ride out their depression and come to terms with their lives.

Hornby is a writer of great feeling and warmth, and it is heartening to see him address mortality, the treatment of which, as Thomas Pynchon reminds us, is the ultimate arbiter of what we call "seriousness" in literature. However, it has to be said that he does so here with mixed results. Some, though not all, of the problem is with the dizzyingly high-concept premise. Four misfits meeting as they're about to jump off a rooftop -- it makes a great movie pitch and, in a good director's hands, with emotive close-ups and the right soundtrack (a couple of suggestions for which are included here, interestingly), could well work on the big screen. But in book form, it simply fails to convince. The first 50 pages are crippled by implausibility. "Don't do it, Nick!" you want to cry; but he does.

When his quartet comes down from the roof, Hornby seems on surer ground. There is an excellent scene at an art-school party, where Jess's mendacious boyfriend is cornered. (Actually, he's punched by Maureen and kicked, a few pages later, by Jess: This is a direct swipe from the movies, women beating up men being Hollywood shorthand for catharsis.) However, after that, Hornby appears at a loss for what to do with them. They agree to postpone their existential decision till Valentine's Day, then again till the end of March. Although the dialogue is consistently funny, there's a nagging sense that the characters are merely killing time (instead of themselves) -- which may well be how many of us live our lives but doesn't give the novel much in the way of momentum.

Hornby never seems quite at his ease with his subject. Having raised the issues of depression and suicide, he then devotes much of the book to backing away from them, attempting to retreat to the arenas of social comedy where he feels secure. This is a shame because he is capable of great empathy: "A man who wants to die feels angry and full of life and desperate and bored and exhausted, all at the same time," JJ the disillusioned guitarist tells us. "He wants to fight everyone, and he wants to curl up in a ball and hide in a cupboard somewhere. He wants to say sorry to everyone, and he wants everyone to know just how badly they've all let him down." But the only real moment of darkness that is allowed to enter the narrative comes from a taxi driver from an unnamed African country whose family has been slaughtered; the narrators feel guilty, give him a "very large tip," and he passes on into the night.

Although it pulls its punches, A Long Way Down is high on charm and frequently hilarious. Maureen left me cold -- Hornby plays her situation for all the pathos it's worth, to the point where it comes to seem relentless -- but the other characters are sympathetically drawn, and in Jess he perfectly captures the teenage cocktail of antisocial horribleness and quixotic idealism. Any time the plot is on the verge of sentimentality, she is on hand with an acidic quote: shamelessly grilling Maureen about her sole sexual experience or asking whether she'd like the others to kill her disabled son for her: "It's not like he's got much to live for, is it? If he could speak, he'd probably thank me for it, poor sod." In fact, Jess leads most of the action, with the other three plodding after her; and although Martin grumbles (after another of Jess's bonding exercises has gone awry), "That's the thing with the young these days . . . they watch too many happy endings," you realize that that, after all, is what you're going to get.

Perhaps it's telling that Hornby's foray into adult themes should end up dominated by a teenager addicted to movies; but although overall the novel feels like a missed opportunity, it misses with such bonhomie and good-heartedness that you're tempted to forgive the book its flaws, and even muster a cheer for the happy ending. *

Paul Murray's first novel, "An Evening of Long Goodbyes," was shortlisted for the Whitbread Prize.