Until midnight tomorrow you can watch a new three-act musical from Joss Whedon (“Buffy the Vampire Slayer,” “Firefly”) online. It is the story of Dr. Horrible (a sensational Neil Patrick Harris) and his nemesis, the very manly Captain Hammer (Nathan Fillion of “Firefly” and “Waitress”). And of course there is a girl, Felicia Day as Penny.

Mary Elizabeth Williams of Salon says:

So you’ll have to forgive me if I lapse into slavish overpraise here for Dr. Horrible’s Sing-Along Blog, the utterly brilliant singing, dancing, Web-only action-adventure you and your brothers have cooked up. I hope it doesn’t sound like hyperbole when I say that Dr. Horrible is better than kittens and sunshine and cheese. Doled in three tantalizing, quarter-hour-long nuggets (the first two went up earlier this week, the conclusion debuts July 19), “Dr. Horrible” stars an impeccably dorky Neal Patrick Harris as a would-be supervillain who pines for his lovely laundromat crush, Penny (a luminous Felicia Day), while battling nemesis and rival Captain Hammer, a musclebound jerk played with idiot bravado by “Firefly’s” Nathan Fillion. Will Horrible create his freeze ray and earn his way into the Evil League of Evil? Will he get the girl? Tuneful, hilarious and, in typical Whedon fashion, unabashedly tender, the only thing wrong with Dr. Horrible is that the damn thing isn’t a regular series.

It has already inspired responses like this one from a would-be sidekick:

After Sunday night, it will be available only on DVD, so watch it online while you can! (NOTE: Brief mature material)