Westport News film critic Susan Granger reviews the new movie, “The Lady in the Van.”

Dame Maggie Smith — a.k.a. Dowager Countess of Grantham on TV’s “Downton Abbey” — stars in Alan Bennett’s adaptation of his hit West End play, based on his best-selling memoir, delivering a tour-de-force, Oscar-caliber performance.

After Bennett reluctantly befriended Miss Mary Shepherd (Smith) in 1974, the eccentric British bag lady “temporarily” parked her dilapidated van in his driveway in suburban Camden and proceeded to live there for the next 15 years. A former nun, she explained that the Virgin Mary had advised it.

The celebrated London playwright/screenwriter/actor/author is played by Alex Jennings in a dual role: the Alan Bennett who writes and the Alan Bennett who observes and probes for truth. He engages in a long-running debate with himself about what to do about increasingly decrepit and demanding Miss Shepherd, who smells of urine, feces, raw onions and talcum powder.

“One seldom was able to do her a good turn,” he observes, “without some thoughts of strangulation.”

At the same time, Bennett is dealing with his mother, who suffers from dementia. And it’s only after Miss Shepherd’s eventual demise that he discovers her “secret.” Years earlier, a young motorcyclist had crashed into her van; panicked, she left the scene of the fatal accident before the police arrived, making her technically guilty of a felony and open to blackmail.

A fervent Catholic, guilt-riddled Miss Shepherd often confesses this sin to a priest who repeatedly absolves her. Eventually, he’s pressed to explain: “Absolution is not like the bus pass. It doesn’t run out.”

Director Nicholas Hytner (“The History Boys”) studs this touching, “mostly true” character study with cameos by veteran British thespians Roger Allam, Jim Broadbent, Dermot Crowley, Frances de la Tour and “The Late Late Show” host James Corden.

On the Granger Movie Gauge of 1 to 10, “The Lady in the Van” is a whimsical, poignant 7, serving as a showcase for Maggie Smith as the filthy, self-centered vagrant to whom “feelings of gratitude, humility and forgiveness were either foreign to her nature or had become so over the years.”

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