It's a depiction of a true story which was happened in California in the late 20's. To put it in a single sentence it is the struggle of a mother to get back her son which intertwines the actions and the lives of LAPD officers, City reverend, Staffs and Patients of the Psychotic department of the government hospital and a psychopath who abducts and kills children. It would have required 2 Hours 15 Minutes for any good director to put all of these in a well structured manner. At some points in the movie we think that its gonna end in 10 minutes only to be introduced to a new plot every time. In one scene, the LAPD captain mentions that everyone loves happy ending. Well, we are rather in for an emotional ending which would make people happy as it ends with the word which keeps this world moving - Hope.
Well, it could possibly act as a lesson to the normal family that there had existed people in their own land who had loved their children so much that they were ready to sacrifice their life. When we see a movie, we unconsciously tend to rate it based on its relevance to the present and to the future. We don't really care about our past. So when someone ventures to make a movie based on a true story happened 80 years back, the audience would expect the movie to show things upon which we have improved and things that still remains the same. Which duly is the purpose of this movie. Well, here the things that we have improved upon are the discovery of DNA and the policies and laws that pertain to normal citizens. And things that remains the same are the psychopath killings and the policies and laws that don't pertain to normal citizens.
It does discuss and analyze a lot of issues. For instance, how the psychopath believes that he can escape hell by being good and candid. When Mrs.Collins asks him whether he killed her son or not, he refuses to say anything and upon her insistence he says 'I don't know'. When we look back it can mean that he didn't find Walter after the three children escaped from his ranch. But then, we cannot be sure as 'I don't know' can also mean that he actually was refusing to answer her question. And then comes the child who pretends to be Walter. When he is being joined with his true mother, he says something which means that LAPD had in fact asked him to act that way. Again, It shows how naive journalists had been during those times which gives us a chance to boast the current journalists who expose scams and scandals.
Overall, Changeling makes us happy, sad and emotional. And it gives us an opportunity to boast our achievements in the past 80 years. It also wants us to review things that remains the same. Not a must-see but you gain some good if you see it.