If you're a serious fan of Maggie Smith or the English playwright Alan Bennett, you may enjoy The Lady in the Van. For the rest of us, though, director Nicholas Hytner's "mostly true" biography is a thing to be endured.

The Lady in the Van began as a stage play by Bennett based on a time in 1970 when an old lady parked her van in his London driveway and didn't leave until her death 15 years later. Bennett adapted the screenplay himself, and the Royal Shakespeare Company's Alex Jennings plays not one but two younger versions of the author. There's the Bennett who rides his bicycle to the theater, drinks tea and talks to his neighbors, and then there's the writer version, who sits studiously at their work station and occasionally bickers with his other half. It's a suspiciously similar structure to a better film called Adaptation (2002), in which the screenplay's author, Charlie Kaufman, writes himself into the script and invents a twin brother as a shadow alter ego. The gimmick works in Adaptation because the invented twin has a personality distinct from the real author. Donald is confident, goofy and marketable in all the ways Charlie isn't, and that makes for revelatory moments between them.

Here, Bennett invents a writer persona as a thing distinct from his regular self, but it's not, really. The two Bennetts dress the same, have the same temperament and disagree on only the most minute of details. For example, is Bennett's decision to let the old woman park in his driveway a matter of British politeness or timidity, and, what's the difference? Perhaps I'm just jealous of Bennett's self-described talent for such compartmentalization. In my experience, the side of me who pumps gas and stands in line has no reprieve from the side of me writing this review; we are hopelessly, inextricably linked.

Now, about this lady in the van we've heard so much about. The film often treats her like a metaphorical extension of Bennett, but in fact, she's based on a real person, named either Mary or Margaret, and of course she wasn't always homeless. An accident at the beginning of the film has her on the lam, and she might have had a few screws loose before that. But she was an educated woman once with a rich history we learn about through the film's series of plotless, ambling turns.

Overall, Mary's a cantankerous, proud and plainly delusional person who I was never able to fully like, but on the strength of Smith's performance I managed to eventually get used to her. Smith has been old my entire life and always excellent; her comedic turn in Sister Act as the nun who doesn't like change is so good it belongs in a museum. It's more of a stretch to buy the matriarch of "Downton Abbey" in such a lowly position, but mostly she gets away with it. And if you've forgotten that people who live in vans smell, don't worry, this film will remind you of it in a joke that isn't funny at least half a dozen times.

If I sound like I'm being hard on this film, well, somebody has to be. The Lady in the Van has gotten almost entirely positive reviews, but I get the feeling they're the result of a whole lot of faint praise. The movie has a sweet message about how we should treat poor and elderly people, but that lesson comes with a predictable, cloying and self-congratulatory delivery I am unable to forgive.

The Lady in the Van opens Fri., March 11, at the Roxy.