Trailer from “Dr. Horrible’s Sing-Along Blog.”

“Dr. Horrible’s Sing-Along Blog” is a short web movie to be released in three parts. The first act showed up today, with follow-ups becoming available on July 17 and July 19. The free streaming videos will remain available only through Sunday, July 20, when they will “vanish into the night, like a phantom.”

According to Whedon, this idea was the brainchild of many conversations about finding alternative funding sources for projects for writers affected by the winter strike:

Aided only by everyone I had worked with, was related to or had ever met, I single-handedly created this unique little epic. A supervillain musical, of which, as we all know, there are far too few.

The idea was to make it on the fly, on the cheap – but to make it. To turn out a really thrilling, professionalish piece of entertainment specifically for the internet. To show how much could be done with very little. To show the world there is another way. To give the public (and in particular you guys) something for all your support and patience.

The brief free run of the film will be followed by an iTunes download charge of $2 per act or $4 for all three episodes. Whedon is hopeful pirating is minimal. (“We have big dreams, people, and one of them is paying our crew.”) The project is very Web 2.0 savvy, leveraging Twitter, Facebook, Wikipedia, and other channels. Dr. Horrible has been released as an online comic. Reportedly, there will also be action figures. The marketing campaign ends with a DVD with “amazing extras” that hints at being innovative as well.

The plot involves Neil Patrick Harris (forever and ever TV’s Doogie Howser, MD) as a supervillian who is constantly thwarted by the local do-gooder, Captain Hammer, played by Whedon favorite Nathan Fillion (Firefly, Buffy). Horrible is conflicted because of his infatuation with a community activist, Penny (Felicia Day, another Buffy veteran). The film is a family affair, co-written by Joss and two half-brothers, Zack and Jed. Joss and Jed are credited with the musical numbers.

Act I is just under 14 minutes and paints the picture—through a couple musical numbers—of Dr. Horrible’s frustration. He wants to make the big time with the most evil supervillain league around but also has a personal agenda to make time with the cute girl he sees at the laundromat. Captain Hammer foils his initiation heist and seems to capture the girls’ heart in the process. Horrible sums up his emotional state in one word: “Balls!”

UPDATE JULY 29: The entire 42-minute episode is available on Hulu.