“I don’t think films about elderly people have been made very much.” — Maggie Smith

Director Nicholas Hytner’s new film, “Lady in the Van,” opens with the line “A mostly true story.” Of course, most stories, social, political, are “mostly true.” It’s rare that an entirely true story about anything or anyone can make it to the screen. Warts abound on the faces of all villains and heroes.

Even “mostly true” stories need a little juice. Hytner’s film, about someone who used to be called a “bag lady” and is now gently referred to as “transient lady,” uses the juice, both acidic and sweet, of one of our favorites: “Downton Abbey’s” juiciest character actress, the inimitable Dame Maggie Smith.

Remember, this is the Maggie who played Desdemona to Laurence Olivier’s black-faced Othello, and stared down Ian McKellen’s Richard the Third. Proceed cautiously.

Here, for our delight and fare, we get Lady Violet Crawley without makeup, but with the same practiced bot mots and flesh piercing stares.

She comes to us this time all done up in Dickensian rags, and with a face full of lines and weariness, and living in a crumbling old van. No mention is made of where she got the money to buy it, or how on Earth she got a driver’s license. She may have stolen both.

“Lady” is based on writer Alan Bennett’s 1999 play of the same name. The film starts with the introduction of Bennett in a strange dual role: Bennett, sitting at a desk writing most of the time, while his twin, alter ego is walking about and chastising him for this and that. We don’t know what to make of that, but we’re sure it will be explained. I don’t think it ever was, not clearly.

Then, Lady, I mean, Ms. Shepherd, arrives, rumbling up the street and parking on the lane in front of Bennett’s newly acquired cozy little cottage.

Ms. Shepherd lives inside her van, like Pinocchio swallowed by the whale, finding only a small space under the weight of tons of newspapers and assorted debris.

The neighbors, a clutch of upper middle class snobs, of course find her annoying, like a potato blight or space alien, in a smelly saucer who has landed on the wrong planet.

Bennett’s Bennett is sort of an intellectual geek who can’t say no, even to birds who dump on his new car. When, on Day One, Ms. Shepherd asks to use his bathroom, listed here as “lavatory,” and he lets her, we shout, “Don’t, don’t do it … she’ll …” and of course she does. We know where this is going. We know she’s going to enlarge her book of daily requests and become a pain in his Dockers, and so she does.

The neighbors are all aghast but for one or two, who, feeling the pangs of guilt, bring her casseroles and gifts, which she snatches with a snarl and then slams the rotten door in their faces. Feeling sorry for her and concerned for her safety, Bennett allows her to move her van into his snug little driveway, where she stays (spoiler alert) for 15 years, until finally she … I’ll leave that for you.

“Lady” is a typically British slow starter and crawler, and then eventually blooms into a sweet story about a sweet old pain in the arse.

But it’s Maggie Smith, for God’s sake. You can’t go wrong when casting Maggie Smith.

I’ll bet that David Lean would have cast her in Omar Shariff’s role in “Lawrence Of Arabia,” if he could have gotten her.

She’s so delightful even when she’s annoying, and her Ms. Shepherd is most of the time.

Throughout the story, efforts are made by the neighbors to transfer her somewhere else, anywhere else.

But Bennett grows fond of her, as though she’s a stray cat with amusing quirks and sweet eyes. I won’t give the ending away any more than Bennett could give Ms. Shepherd away.

J.P. Devine is a former stage and screen actor and the author of “Will Write for Food.”