It’s been so long since Peter O’Toole did his greatest acting — decades, actually — that it’s easy to forget (or to never know in the first place) how good he is. He has the same kind of magnetism that Marlon Brando had, where even in a bad movie, you can’t take your eyes off him. What’s more, he makes it look effortless, like he’s just being himself, twinkling eyes and lilting British accent and all.

In “Venus,” he almost is just being himself, and what do you know, he excels at it. He plays Maurice, a frail old London actor who’s had a long life of bit parts and occasional leads, now content to play a comatose grandfather in a soap opera, or whatever other one-day parts he can get. Convivial and soft-spoken, he loiters at a neighborhood cafe each day with his friend Ian (Leslie Phillips), also an elderly actor, where the two trade pills and look forward to their afternoon drinks.

Soon Ian’s great-niece, Jessie (Jodie Whittaker), comes to live with and take care of him. Jessie is a sullen, slack-jawed 20-ish layabout with no interest in nursing the old man; she wants to be a model. Ian is terrified of her. Maurice finds her youth and honesty refreshing. Maurice and the girl become unlikely friends, with Jessie appreciating the attention Maurice pays her and Maurice enjoying the opportunity to wander around town with a pretty girl on his arm.

There isn’t much more to the film than that in terms of plot, though there are a couple of other angles worth mentioning. One is Maurice’s prostate-cancer scare. “I am about to die and I know nothing about myself,” he says with his usual articulate grace and gentle candor. The other is Maurice’s relationship with his ex-wife, Valerie (Vanessa Redgrave). After what we gather were some years of selfishness (on his part) and rancor (on her part), they’ve now settled in to a comfortable friendship that suits them both.

It’s elegantly written by Hanif Kureishi and smartly directed by Roger Michell (“Notting Hill,” “Changing Lanes”), and the supporting cast is effective, but the film is entirely O’Toole’s. Every adjective I can think of to describe the movie applies to O’Toole’s performance, too: funny, endearing, good-natured, classy, and erudite. Maurice (and, I suspect, O’Toole) is the sort of old-fashioned man who likes his whiskey, who calls women “my dear,” and who uses the word “shall” unselfconsciously. His indefatigability despite age and failing health is utterly charming, as in this exchange with Jessie while the two are out one day:

“Are you tired?” she asks.

“Oh, yes,” he replies.

“What should we do?”

“Carry on!”

Carry on, indeed! What else is there? To just stop and give up? Nonsense, my dear, nonsense. “Venus” isn’t terribly deep or complicated, but it does do one important thing: It sends you out of the theater smiling.

B+ (1 hr., 35 min.; R, a lot of harsh profanity, a little sexuality, brief nudity.)