Watchmen is a classic graphic novel. The movie didn't do it justice but to be fair, I don't see how it could within the two or three hours movies usually take; lots of interwoven stories, jumping between different eras, etc. The series is a sequel after the events of the movie/graphic novel, to the best of my knowledge it's not based on any previously published material. It's interesting that Alan Moore, co-author of the original, is not credited at all in the series; the other co-author, Dave Gibbons, is part of the team putting on the series. I suspect Moore refused to have his name associated with it, he'd said the graphic novel was 'unfilmable'.