In a nutshell: 
      A boy goes missing in LA in the 1920s. Some six months later, he is 
      returned to his mother, and the heartwarming mother and son reunion is 
      touted as a brilliant example of painstaking police work. Only one slight 
      problem: the boy who returns to her is not her son. He is an imposter. She 
      keeps trying to convince the police that they need to continue the search 
      for her real son, but the police are not willing to undermine the good PR 
      they established with their "successful" solution to the case, and are not 
      willing to face the bad publicity that would result from having that 
      solution undermined. The mother keeps amassing evidence that the boy is 
      not her son, and the corrupt police have no recourse other than to use an 
      obscure legal procedure to have her committed to an asylum, for she must 
      be mentally ill if she will not accept her own child! The police have 
      gotten away with similar machinations in the past, but fail this time 
      because of the efforts of a crusading minister whose life is dedicated to 
      the relentless exposure of police incompetence and greed. 
      True story. And the screenplay stays as close as possible to the facts 
      of the case.
      Although Changeling is considered an important movie (8.1 at IMDb and 
      two Golden Globe nominations), and features both a major star in Angelina 
      Jolie and an esteemed director in Clint Eastwood, I'm not going to devote 
      as many words to it as I normally would. The reason is that the
      
      Wikipedia Page for this film is absolutely superlative. In terms of 
      background, it says everything I would have said and more.
      In terms of evaluation, I have only a few minor points:
      
        A) I liked the overall look and feel which Eastwood gave to the 
        1920s, and I was impressed by the the sparse, melancholy score, which he 
        wrote himself. He's a talented man.
        B) Overall, the movie doesn't work for me. The fundamental reasons 
        are two-fold:
        1) Most important, the film goes on about half an hour after the 
        story I described above has ended. The final act is about a serial 
        killer who may or may not have killed the real missing son, and whose 
        story is basically just an epilogue which has only a peripheral (and 
        tenuous) relationship to the story of the mother and the police. In my 
        opinion, the final 30 minutes (or so) could have been covered with a 
        single word slide. We do want to know what happened to the real son, and 
        we have to know the motivations of the imposter, but the serial killer's 
        back-story is one of those "meanwhile, in another movie ..." 
        digressions.
        2) Changeling breaks Scoop's first rule of biopics, which goes 
        something like this: "If we did not know in advance whether the story 
        were true, but simply watched it cold, would we still find it a good 
        movie? A factual film must be able to exist on its own, without leaning 
        on the crutch of truth." I watched this film and did not know how 
        painstakingly accurate it was. Therefore, in addition to the 
        anti-climactic nature of the film's final act, I also objected to 
        elements of plot and characterization which I considered melodramatic, 
        unrealistic, and lacking in nuance.
        Having established that, I have to admit that I did study the details 
        of the story after I watched the film and, having done that, I found it 
        completely shocking and terrifying that such a thing happened in the USA 
        in the 20th century. Little had I known that reality was unrealistic in 
        this case. I then realized, in retrospect, that the film did effectively 
        drive home the emotional core of that shock and terror.
      
      So there you have it. If you watch the film knowing that it presents a 
      fair and accurate account of a true story, and are interested in such a 
      story, you will probably be profoundly moved and will undoubtedly lose a 
      small portion of whatever faith you have in the essential goodness of 
      mankind. If you watch it cold, as I did, you will likely find it to be 
      melodramatic and unrealistic, because sometimes reality is stranger than 
      fiction can dare to be.
      You make the call.