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

Mary (or is it Margaret?) Shepherd was a pianist. And then she was a nun. And then… well, it’s not clear just what happened next. But by the time we meet her, in Alan Bennett’s memoir-turned play-turned movie, Miss Shepherd (played by Maggie Smith) is a filthy and irascible old woman who sells religious pamphlets that she might or might not have written herself (“Let’s just say they’re anonymous,” she snaps) and lives in a van that she parks on the streets of a gentrifying neighborhood in the London of the 1970s.

Alan Bennett (played in the film by Alex Jennings), for his part, is himself: a writer who lives in a house in that same gentrifying neighborhood, and who becomes one of a number of socially-conscious residents who are alternately repelled by Miss Shepherd or concerned for her, who wish that she would accept with grace the help they offer her (she doesn’t) or, preferably, that she would just go away (she doesn’t do that, either). Then the parking regulations change, rendering Miss Shepherd’s situation yet more precarious. What will she do? What will he do?

What they do is agree that Miss Shepherd will park her van in Mr. Bennett’s driveway, “just for a few months,” until she can make arrangements for the longer term. And then the few months become the longer term: fifteen years, in fact. During these years Miss Shepherd becomes ever more enfeebled and impossible to tolerate, Mr. Bennett’s elderly mother does the same, and Mr. Bennett himself wonders whether he is failing both of them: his mother, by putting her in a home, and Miss Shepherd, by allowing her to make her home in his driveway.

Mr. Bennett is of two minds about all of this, a fact that he dramatizes in both the stage and screen versions of his story by having himself played by two actors (on the stage) or by one actor playing both roles (on the screen). On the one hand, he is a writer, always on the lookout for interesting material, and Miss Shepherd is nothing if not interesting. On the other hand, he is a single man, decidedly skeptical about the complications that come with relationship, and Miss Shepherd is pure complication. Can he bring these parts of himself together? Does he even want to?

Miss Shepherd herself is a far from integrated personality, as the uncertainty about her first name suggests. Is it Mary? Is it Margaret? Why doesn’t anyone seem to know for sure? Why does she fly into a demented rage whenever the neighborhood children practice their instruments? Why does her devoutly Roman Catholic piety seem to bring her agony rather than consolation? How can a woman who for decades has answered nature’s call with plastic bags still be concerned enough about appearances to try to cover up the stench with lavender-scented powder?

Miss Shepherd’s eccentricities, coupled with her delusions (the Virgin Mary tells her where to park her car) seem clearly to place her on the far side of the line that separates sanity from mental illness. But is she the only one over there? Psychoanalyst Christopher Bollas, describing what he calls “normotic illness,” writes, “If psychotic illness is characterized by a break in reality orientation and a loss of contact with the real world, then normotic illness is typified by a radical break with subjectivity and by a profound absence of the subjective element in everyday life… [I]f the psychotic has ‘gone off at the deep end’, then the normotic has ‘gone off at the shallow end’.”

Who in this film meets this description, the normotic whose rigid avoidance of subjectivity forms a foil to Miss Shepherd’s life of psychotic fantasy and hallucination? A hint: it isn’t Alan Bennett. Granted, the author’s dual self-portrait displays marked ambivalence toward the world of interpersonal relationship. In the only glimpse we get of his bedroom, it is austerely furnished with virtually nothing but a single bed. And in a scene late in the film, a social worker refers to him as Miss Shepherd’s “carer.” “Do not call me the carer!” he erupts. “I do not care, and I do not care for!” Only, obviously, he does.

On one level, The Lady in the Van is a lightly comedic portrayal of a crazy old lady who parks her van in a playwright’s driveway and stays there for fifteen years. On another level it is about the difficulty of holding together disparate aspects of oneself — the observing self vs. the experiencing self, the altruistic self vs. the self-serving self, the generous self vs. the judgmental self, the self that longs for life and connection vs. the self that is terrified of these things or fears it does not deserve them.

And on yet another level, it is about the power of relationship, and the ways in which people we do not choose — like our neighbors — come barging into our lives and prove both maddening and transformative, perhaps in equal measure. “One seldom was able to do her a good turn without some thoughts of strangulation,” Mr. Bennett says, wryly, of Miss Shepherd. And yet in the final scenes of this film are glimpses of the changes that each has wrought in the life of the other.