Let me just start off saying, you have to watch this movie with a sense of humor, a love of super heroes and you can deal with musicals....yes, because this move is all of the above. Wikipedia refers to it as a tragicomedy (I was unaware there was such a word until I saw this) and well it is a good summary of this movie.
I don't want to say much of the plot as I don't want to ruin it and would rather those interested to have the experience....
The most amazing part of this movie is the cast...
Neil Patrick Harris as Dr. Horrible, a rising super villain that wants to join the Evil League of Evil and hosts his own blog telling his story to his audience.
Nathan Fillion as Captain Hammer, better known as the Captain from Serenity among other things. He plays the shallow, ego driven super hero that is the nemesis of Dr. Horrible.
Felicia Day as Penny the girl between the hero and villain...she is better known to me as being on the Guild among other things including Eureka, she is one of those actresses that when you see her you know her but she does not appear in things that are the common movie.
What is most interesting is this movie is done in three acts of about 14 minutes each, so it is not a long movie and from a film point of view was very well done. It took a moment to get used to the concept but once I got into it I was rooting for the villain to win out!
Oddly, I watched this on Netflix, but Youtube has it free!