I'm unfamiliar with the play upon which the movie Doubt  was based, but I'm not sure it really matters at all. The production is a fairly static drama, without any real action or cinematic qualities. All conversation and gestures.
I'm not sure why, but I thought I was going to dislike this movie. I was not raised in a religious family, and I have precious little patience with dogma and the hokus-pocus of elaborate ceremony. What is it that Frank O'Hara said about the Catholic Church?--"only a very mediocre introduction to cosmic entertainment"? Whatever.
This story is set in a Catholic middle-school in the Bronx. It was filmed on location, and has all the right gritty dreary feel of the place. A lower-middle-class mostly Irish working family kind of neighborhood. Priests and altar boys, the stereotyped strict Mother Superior principal--it sounds like a cliche situation, and it is.
But that's where the predictable part ends. You might assume--as I did--that Philip Seymour Hoffman (as the Priest Brendan Flynn) was going to dominate this film, since he's the "suspected" child-abuser around whom the action revolves, especially given his credentials. But he's completely overshadowed by Streep (the school principal), Amy Adams (as an idealistic, naive young instructor), and Viola Davis (the mother of a boy suspected of having been seduced by Flynn). The plot comes down to a siege between Sister Aloysius Beauvier (Streep) and Flynn (Hoffman).
The "doubt" of the title is our growing suspicion that Sister Aloysius is correct in her belief that Flynn--with a history of repeated short appointments--is a closet child molester. The recent high-profile court cases in New England over the last decade have fueled public interest in this subject-matter.
But the really great thing about this movie is Streep's classic, mature performance as the frustrated, driven, demonic, frost-bitten old crone. Confined by the nun's habit, the dark hooded bonnet surrounding her face, she manages, just with her expression, her hands and a few sharply controlled bodily gestures, to suggest a full range of passion, frustration, triumph, and grim predatory assault. Her performance here is probably as good as anything from the legitimate stage. I doubt Katherine Hepburn, or Betty Davis, or Ingrid Bergman could have done half as well. Streep has always had a sort of dry Nordic posture, but she's never before had the opportunity (well, maybe The Devil Wears Prada is the exception) to exploit this side of her acting resource.
Her Sister principal is hard as nails, without a single redeeming molecule of pity or sympathy. Even in the end--having triumphed over Flynn, and sent him packing after he's capitulated to her threats and intimidation, when, in the very end, she pulls a quick little seizure of tearful self-doubt--her self-possession is so impressive that you really don't buy it. It's just a bit of dramatic editorializing meant to soften her monolithic composure.
Amy Adams is almost as good as the obsequious little Sister James, meekly asserting her virtuous good intentions against the intimidating Streep.
I was reminded of The Scarlet Letter--the same dark, swirling gossip, the evil angels of righteousness cloaked in authority, filled with creepy sexual denial and loathing.
I won't give the best part of the plot away, because it's truly a surprise.
This isn't a great movie, perhaps, but the performances of Streep, Adams and Davis are among the best instances of pure acting of the last 20 years. It's the kind of movie you watch again, just to savor those "moments" when the drama is brought repeatedly to a boil.