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Maggie Smith Alex Jennings

This photo provided by courtesy of Sony Pictures Classics shows, Maggie Smith, left, as Miss. Shepherd and Alex Jennings as Alan Bennett in the film, "The Lady in the Van." (Nicola Dove/Sony Pictures Classics via AP)

Everyone knows Miss Shepherd (Maggie Smith). She’s the old lady who lives in a van, and whose personal hygiene severely limits her popularity. But as long as she parks her vehicle somewhere other than on their property, the folks in her spiffy London neighborhood are fine with her.

One of them is writer Alan Bennett (Alex Jennings), who lives so much inside his head that he imagines himself to be two people: one with pen in hand, the other focused on the world outside their window. Inevitably, Miss Shepherd attracts the attention of both — not necessarily to their benefit.

Despite her situation, she’s excellent at reading people. And it’s not long before she’s insinuated herself into Bennett’s life. Her masterstroke: Parking her van in his driveway. And remaining there.

Bennett’s not happy about having an unwanted guest, but he’s also too polite to get rid of her. As years go by, the writer and the squatter find themselves involved in an uneasy friendship.

Through it all, Miss Shepherd receives periodic visits from a man named Underwood (Jim Broadbent), whom she pays to keep silent about an incident from her past.

Based on a true story, “The Lady in the Van” is a well-acted but somewhat wearying exercise in British whimsy. Working from a screenplay by Bennett, director Nicholas Hytner takes his time getting things moving — and seems overly dependent on his actors to keep things interesting.

That said, Smith — of “Downton Abbey” and “Harry Potter” fame — delivers a performance that’s just a bit grittier than might be expected, revealing the pained human being inside the eccentric-old-lady shtick. And Jennings (“The Queen”) is slyly appealing as a man who comes to realize that, to his astonishment, he’s still capable of change.

“The Lady in the Van” doesn’t pretend to be anything more than a pleasant diversion. And on that level, it succeeds.

What “The Lady in the Van” • Three stars out of four Run time 1:43 • Rating PG-13 • Content Brief unsettling image