This has probably happened to everybody: You’re in a social situation, talking to someone you barely know. They launch into a long story, and it’s mildly interesting but never so interesting as to distract you from the question: “Why are you telling me this?”

“我老了School” is the movie documentary equivalent of that situation. Set in Scotland, it tells the story of Brian MacKinnon, who’d flunked out of medical school years before. He knew he couldn’t get back into medical school under his real identity. So, at 32 years old, he changed his name to Brandon Lee, re-enrolled in high school and passed himself off as a teenager.

What makes this story particularly odd is that “Brandon” enrolled in the exact same high school that he’d graduated from over a decade before, and yet, none of his old teachers recognized him. For someone needing to maintain a low profile, he allowed himself to become popular, even starring in the school production of “South Pacific.” And he invented an elaborate history for himself, including a dead opera singer mother and Canadian citizenship.

All this is reasonably, vaguely interesting, but the documentary’s telling of the story is hampered in a number of ways. It took place more than 25 years ago, and even though this is the first that most of us are hearing of it, it just doesn’t feel fresh. Lively music is piped in throughout to make it all seem sprightly and new, but a quality of staleness clings to the material.

Another problem is that there’s very little actual footage from Brian/Brandon’s time in school. To illustrate the action, the movie relies on rudimentary animation of the “Beavis and Butt-Head” variety, and it’s just not a pleasure to look at.

Finally, though Brian MacKinnon himself was interviewed at length for the documentary, he refused to let himself be filmed. Director Jono McLeod comes up with an ingenious solution to this problem. He brings in actor Alan Cumming to lip-sync McKinnon’s words.

Cumming is an appealing actor, who, back in the 1990s, was talked up as someone who might play MacKinnon in a dramatic feature. Cumming approaches his task in “My Old School” with seriousness and gives a real performance. But all the same, something is lost by not having the actual MacKinnon onscreen.

As it stands, MacKinnon remains unknowable, so what other possible understanding is to be gained from watching his story? Perhaps “My Old School” might have worked as an investigation into the effect his deception had on the community. Only problem: There wasn’t any real effect worth mentioning.

Or perhaps the story might have spoken to larger truths. For example, it might have worked as a rumination on the impossibility of remaking the past — and on the longing to go back and make things right. But no, MacKinnon is too cold-blooded to inspire poetic rumination, and his story is too specific and limited to have larger application.

In the end, “My Old School” is a well-made documentary that succeeds in most ways but that starts to crumple in the face of a single question: Who cares?

故事发生了。美好的。这不是无聊的。但是你为什么告诉我这个呢？

L“My Old School”:Documentary. Starring Alan Cumming. (Unrated. 104 minutes.) Opens Friday, July 29, at the Opera Plaza Cinema, 601 Van Ness Ave., S.F.landmarktheatres.com