What a mighty fortress Peter O’Toole is. Battered and blown-out by the ravages of time and his own prodigious bad habits, the actor teeters headlong into his mid-70s as a glorious ruin of a man, held intact by his devilish charisma and the impeccable architecture of his cheekbones. A bad boy’s bad boy, Mr. O’Toole has outlived most of his legendary pub mates — boozy rogues like the Richards (Burton and Harris) — and simmered into a mellow old age. His physical presence, elegant and somehow eroded, blue eyes still crackling with mischief and desire, may put you in the mind of a great jazz saxophonist who achieves an autumnal grace amid diminishing chords: He’s all essence.

Mr. O’Toole barely has to shift a gear for his role in “Venus,” a romantic comedy as valedictory lap, from the writer-director team of Hanif Kureishi and Roger Michell (“The Mother”). He plays Maurice, who is basically a less-famous version of himself. Shuffling around Kentish Town, he spends his fading days bantering with Ian (the splendid Leslie Phillips, another spirited old crank). They relive past glories, mourn the passing of friends noted in the Guardian, and swap medications at the corner diner, popping random handfuls of uppers, downers, whizzers, and bangers for their entertainment value. Inevitably, they nap a lot.

Sometimes, Maurice takes a thankless bit part in a TV production, typically expiring on an operating table, and uses the cash to prop up what remains of his professional dignity.

Maurice is entirely shameless and shines so brightly in his own self-regard that he can’t resist any temptation to swan about. When a bout of prostate cancer saps his manhood and underscores his mortality, the wobbly ladies’ man yearns for one last hurrah. The inspiration is the sudden arrival of Ian’s grandniece Jessie (Jodie Whitaker). The teenager has been sent to help care for Ian, but it’s immediately clear she’s been cast out by her own mother for being a massive nuisance.

Jessie is white trash — a “chav” or a “slag” in British parlance — a petulant nightmare munching chips and burping wine coolers while lounging around in a shapeless pink tracksuit. Maurice sparkles at the sight of her. Jessie takes him for a dirty old creep. Ah, romance.

Mr. Kureishi, who has been working at the seams of England’s patchwork social fabric since his debut with “My Beautiful Laundrette” 20 years ago, has built his career on characters who let passion vault them over the divides of race, class, gender, and age. He continues to do so in “Venus.” Even inclined toward transgression, the screenwriter is perhaps the last among his peers anyone would expect to tackle something as nakedly sentimental as this: “Grumpy Old Men” by way of “Pygmalion.” But the specificity of the writing, by turns brutal and tender in its unsparing candor, and the delightful complexities of the performances, can really grab you up.

Mr. O’Toole is the most disarming of actors, even with spittle flecking at the corner of his mouth. Despite playing Maurice in the character’s weakened state, he makes what happens between him and Jessie fully believable. Ms. Whitaker is sheer piss and vinegar. Yet, as this bratty object of desire tentatively yields to the influence, cultural and, um, alcoholic, of her improbable swain, she blossoms. There is a requisite revelation of youthful tragedy, and a thirst for a kind of redemption. This, even as Maurice takes every opportunity to get her naked, christening her Venus. This sets up scenes of both slapstick — an ill-fated audition as a model for a group of amateur painters — and poignancy, as Jessie makes a particularly intimate truce offering after apparently breaking the poor man’s weary heart. What might seem vulgar on paper becomes painfully touching.

Though the romance, chaste as it is, creates dramatic tension, the pulse of the movie is constant with Mr. O’Toole. When Maurice calls on his ex-wife (Vanessa Redgrave, in a rich performance as rich as it is small) and seeks, in any small way, to atone for his sins against her, she’s having none of his sorrow. But she loves him anyway. Their brief scenes are so eloquent, true, and stripped of varnish that they leave the rest of the film behind. When the tone shifts, at times as uneasily as Maurice’s tilting gait, between four-hanky melancholy and vibrant rude humor, these moments secure the film’s emotional compass.

Mr. O’Toole’s ravaged beauty does the rest.