When I saw this film I knew nothing about the plot, having never read the mid-1980s comic book of which (as I learned later from the Wiki entry) the film is said to be a faithful adaptation. I'm not sure whether this was an advantage or disadvantage; at least I didn't spend my time checking its authenticity and was able to evaluate the film purely on its own merits.
The setting is an alternative 1985 in which the USA has had a history of masked vigilantes (superb fighters but otherwise normal humans) now all retired. There is also one genuine superhero with god-like powers as a result of a laboratory accident, who is now a glowing blue figure known as Dr Manhattan. His existence enabled the USA to win the Vietnam War and gave them a dominant position over the Soviet Union. However, he is becoming increasingly remote from normal human affairs and seems to have disappeared, causing Cold War tensions to become increasingly hot: nuclear war threatens. Meanwhile, someone is killing off the vigilantes and the survivors get together to try to discover what is going on.
Having experienced something of a surfeit of Hollywood superhero movies of late, I had certain expectations. I was expecting light entertainment: a fast-moving thriller with a straightforward good vs evil plot, lots of action and special effects, probably a dash of humour in the quieter scenes, and maybe a touch of thwarted romance. I was therefore rather surprised to discover that, although Watchmen has most of those elements (I can't say I noticed much humour), it also has a lot more. It is a generally slow-paced and indeed very long film, running for over 160 minutes. It is also rather confusing, hopping constantly between the present and the pasts of several of the heroes at different stages of their lives: I wasn't always sure who the younger versions of the characters were meant to be. Overall, it is a rather grim and downbeat film with a pessimistic twist in the ending.
I understand that many aficionadoes of the comic book rate this film very highly. My own view is rather more mixed. There are some good elements and some strong scenes, but overall I suspect that the desire to be faithful to the comic has resulted in a rather messy and confused structure with too much packed into it. It was involving enough for me to watch to the end, but I'm not likely to want to see it again.