Watchmen tells the story set in an alternate version of history, where the U.S. won the Vietnam war and costumed vigilantism/crime fighting - which had become popular for a period - has been outlawed. This is a dark world; where each character is more flawed and twisted then the next.
The impetuous that moves the plot forward is the murder of The Comedian/Edward Blake (Jeffrey Dean Morgan). Rorschach - the one vigilante who refused to give up crime fighting after it was outlawed - begins to investigate, believing that his death is part of a secret plot to kill off all former masked crime fighters. At the same time, the world moves closer to nuclear holocaust as the U.S. and U.S.S.R (the current part of the story is set in the 1980's) engage in an international version of chicken.
Zach Snyder (who also directed 300), tells the story in a highly stylized manor, that some people may find off putting, but which I found to be perfect for this tale. The musical score is one of my favorite soundtracks. I would say that only Black Snack Moan has a better soundtrack.
Watchmen has a complex back-story, which is better fleshed out on The Ultimate Cut version then the theatrical version - which also includes The Tale of the Black Freighter edited into the story, just like it is in the comic book. The Tale of the Black Freighter is a story within a story, that tells of a ships captain, whose shipping vessel has been sunk by evil pirates resulting in him being stranded on a deserted island. He escapes by creating a raft using timber from the ship and the bodies of his deceased crew and heads to save his family, who he believes to be the next next targets. However, his struggles to get home leave him delirious, and he inadvertently kills his own wife, believing her to be a pirate.
This story has many thematic parallels with the main plot of Watchmen, particularly the ending, which asks the age old question of, do the ends justify the means? In addition, the movie also focuses on the reasons why the films "heroes" choose to fight crime. Is if for love of violence? (The Comedian) Fame and attention? (Silk Specter) To soothe dark personal demons? (Rorsach) In this sense, the film, like it's comic book counterpart, is a deconstruction of superhero mythology.
One character I want to discuss is, Adrian Vight/Ozymandias (Matthew Goode), who in the comic book is openly gay. In the Ultimate Cut, Rorschach refers to him as a "possible homosexual". In both comic book and movie, Vight is frequently surrounded by pink or purple triangles and pyramids.
Rorschach, whose mask consists of the changing ink blots from the test that shares his namesake, is also very homophobic, yet I loved the character anyways. The only thing that can forgive me for that, is that he's searingly portrayed by Jackie Earle Haley, who gives the movie's most memorable performance.
Also, Dr. Manhattan/Jon Osterman (Billy Crudep) is the only character with any sort of superpower (other than perhaps Vight who's super smart and fast enough to catch a speeding bullet, which he does at one point). Dr. Manhatten spends much of the movie in the buff, which means every reviewer, male and female, pretty much ends up mentioning his giant blue penis, so I might as well. So I agree, and will admit that it's very blue.
To round this all up, Watchmen is the type of movie that will appeal to people who like their villains dark and their heroes darker. It's not for everyone, and having read the comic book first will certainly aid in comprehension. I count Watchmen as among my favorite movies and therefore, naturally, highly recommend it.