One of the best noir themes is the peeling away of a pretty, false facade to reveal the hard, nasty truth underneath, and, in that theme, Shadow of a Doubt is one of the best examples.
It's early Hitchcock (my favorite Hitchcock film, in fact, probably only challenged by Notorious), and is rife with irony, subtext, and dark humor. The main character, Charlie, is a young girl on the cusp of becoming a young woman in a idyllic American small town. When her beloved uncle and namesake comes to visit, a series of events start to make her wonder exactly who her uncle is and what his connections to the "Merry Widow" murders might be.
What sets this apart in the world of film noir is the fact that it has at its center an entirely innocent heroine. Most of the women featured in noir are either the dangerous dames or the hard luck girls, and whether or not they're up or down, the thing they have in common is that they've all seen the dirty side of life. Charlie has not. She is sheltered in a good family in a small town, and while she longs to get out and see the world, she doesn't really know the first thing about it.
There's also the undeniable subtext of her sexual awakening, which is eerily prompted by her growing realization of her uncle and hero not only as a serial killer, but as a man - and Uncle Charlie's realization that his little niece is now a grown woman. The film may try to deflect the sexual tension mostly onto the relationship between young Charlie and the detective that is following her uncle, but with Hitchcock and his hyperactive sexual imagination at the helm, we should know better than to read that particular plotline as anything but a distraction. As Charlie is confronted with the realities of life beyond her family and childhood beliefs, her new adult understanding of violence, lies, and sex is all tangled up together and presumably leaves her a much different person at the film's close.
There a lot in this film to appreciate - I love young Charlie's younger, know-it-all sister, with her huge round glasses and ever-present stack of books, and her father's ongoing, darkly humorous debate with their geeky, pulp-reading neighbor about the best way to murder someone. Joseph Cotten - my favorite most underrated actor - is perfect as Uncle Charlie. The direction, obviously, is detailed and beautiful. This film is often named the thematic precursor to films like Blue Velvet - which it is, but somehow, a movie like this, which hints rather than explicitly states, is so much more powerful than any other that's tried to take on the same topic of investigating what lies beneath.