Today is the last day you can watch Dr. Horrible's Sing-Along Blog (Whedon, 2008) for free. After today, you'll have to buy it on itunes for $3.99. But for a limited time you can stream it here.
Role-reversal is a common motif in Joss Whedon's work. For example, blond cheerleaders get terrorized by monsters in most horror movies, but Buffy Sommers is a superhero the monsters are afraid of in Whedon's Buffy the Vampire Slayer. Here we get the tragic story of Dr. Horrible in which the evil genius is presented as the sympathetic underdog and his superhero nemesis Captain Hammer is a dim-witted yet pompous boor.
What is fascinating about the movie is Whedon's vision of evil. Dr. Horrible is a geek who talks a lot about undermining the "status quo" in which nice guys finish last. Here evil is seen as the use of violence to make the world into your own image of a better place. Notice that there are two ways this could go. If your image of a better place involves everyone bowing to your slightest wish, you could establish yourself as supreme dictator. This sort of egoistic evil is exemplified in the movie by Bad Horse, the head of The Evil League of Evil. But Dr. Horrible exemplifies a more paradoxical altrustic evil. He wants to take over the world, not so much for personal gain but to create justice. For example, at one point he complains that innocent children might get hurt. In short, Dr. Horrible is a vigilante -- like Batman and other characters usually considered heroes.
But it turns out that Captain Hammer is worse in a certain way. He's not even interested in making the world better. He is himself well off, and so he thinks the world is fine the way it is. He thus exemplifies what you might call egoistic good. The only truly good character -- the altruistic good character for those keeping up with my categories -- is Penny, the love interest. So we hate Captain Hammer and want Dr. Horrible to succeed in defeating him and to "get the girl". And we sympathize with the Doctor's aims while also regretting his methods. This is high tragedy -- note that Penny's true goodness is threatened by the battle between egoistic goodness and altruistic evil.
Dr. Horrible's Sing-Along Blog turns out to be complex and subtle stuff for a musical superhero spoof.