Watchmen takes place ca. 1985 on an alternate Earth, where Nixon is in his 4 term and superheroes have been banned. The superheroes are introduced through an opening montage - we have to infer their powers and passions. In fact, one of the stars, Nite Owl, is only named once in the movie in passing. It is assumed that these heroes and their history is well known to you, which draws you into the universe.
They may be well known to you, if you read Alan Moore's graphic novel that the movie is based on. Personally, I have not - my new philosophy is never to read the book before you watch the movie. It just leads to disappointments. However, I understand that the movie is very faithful to the look and feel of the comic, although the plot takes some hits.
You certainly get the feeling that there's a lot going on that you are missing. Watchmen is densely layered, filled with symbols (like watches), jokes (anti-vigilante riots), and history (a superhero kissing a nurse at the end of WWII). It would definitely reward multiple viewings.
I'll skip the plot - don't want to be a spoiler. Instead, I'll mention the heroes:
- The Comedian, a coarse, brutal Nick Fury type, killed at the start of the movie.
- Rorschach, his mask an ever-changing inkblot, the definition of vigilante.
- Nite Owl and Silk Spectre, both second-generation heroes, both retired, both depressed by trying to live like ordinary citizens.
- Ozymandias, the smartest man on Earth, living openly as a tycoon.
- Dr. Manhattan - naked, glowing blue, able to control all matter, space and time with his mind, but will he be able to keep his humanity?
But the important thing to remember is: all of the heroes are flawed. Some are cruel, some careless, some indecisive, some impulsive. It's clear why society has banned them. But can society live without them?
All of this is aided by a cool visual style like we have seen in Sin City, The Spirit, and some others I haven't seen, like 300. I'm not sure what to call this style - a mix of digital video and CGI, I guess, although there are some other tricks going on: Matrix style slo-mo, perspective distorting lenses, tricks with color, etc. Since half the examples above are directed by Zach Snyder, and the other half Frank Miller, this isn't a director's style. Does it have an established name? The Miller-Moore Effect?