One thing you should not do before seeing “Changeling” is Google the real-life story it’s based on. Knowing the essentials of the plot would ruin the movie, which relies on surprising you — at least for the first 75 minutes — with how crazy it is. I knew nothing about the case beforehand and still found the film barely tolerable. I can only imagine how much duller it would have seemed if I’d already known the outcome.

Clint Eastwood is the director and Angelina Jolie is the star, and both have seen better days. Jolie plays Christine Collins, a telephone operator in Los Angeles in 1928. A single mother, Christine dotes on her 9-year-old son, Walter (Gattlin Griffith), and is devastated when he goes missing one March afternoon and cannot be found.

Five months later, the police locate Walter in Illinois and bring him home by train to a great fanfare of much-needed positive publicity for the LAPD. There’s just one problem: This boy isn’t Walter. He says he is, but surely a mother knows her own son. This ain’t him. But the LAPD, represented by Capt. J.J. Jones (Jeffrey Donovan), who speaks in the rapid-fire patter of an old-fashioned movie gangster, is more interested in closing the case than actually solving it. “You’re a liar and a troublemaker,” Jones tells Christine.

It is clear to us, the audience, that Christine is correct. She gets Walter’s dentist and schoolteacher to verify that this boy is not him. Jones’ response is to throw Christine into the loony bin, which was evidently quite easy for a cop to do in L.A. in the 1920s. And unfortunately, if the police say you’re crazy, you’re crazy. As one of Christine’s fellow prisoners tells her, “The more you try to seem sane, the crazier you start to look.”

Being in a mental institution did Oscar-flavored wonders for Jolie in “Girl, Interrupted,” and there’s plenty of frustrating, why-won’t-they-listen-to-her, “One Flew Over the Cuckoo’s Nest”-style drama to be had here. Meanwhile, back on the outside, a good cop named Lester Ybarra (Michael Kelly), is learning the truth and ruining the movie.

The film is 141 minutes long, you see, and we’re at about the halfway point when Ybarra and the audience discover all the answers about what happened to Walter. At that point, it’s just a matter of filling in the minor details and letting the story play out — which it does in a disappointingly perfunctory, not to mention dragged-out, fashion. It’s a crime film for a while, and then it’s a community-activist film, as Christine protests the LAPD’s treatment of her, aided by a rabble-rousing Presbyterian minister played by John Malkovich.

Jolie’s performance is good, but it’s a thankless role that only requires her to cry for her son for 2 1/2 hours — a repeat, more or less, of what she did in last year’s “A Mighty Heart.” Though we can sympathize with her, it’s hard to truly like a character who’s so single-minded and without nuance.

What the film does (with a screenplay is by TV writer and “Babylon 5” creator J. Michael Straczynski) is set up easy villains and then knock ’em down. Corrupt cops, shady city officials, deranged criminals — even accepting that the basic facts of the film are true, all the realism has been drained in order to make a populist, yay-for-heroes, boo-for-villains story. It’s the kind of movie where the mayor tells his chief of police, “This is an election year! I can’t afford this kind of press!” How could a story as genuinely bizarre as this one seem so generic and rambling on the screen?

C (2 hrs., 21 min.; R, three F-words, brief strong violence.)