Dr. Horrible's Sing-Along Blog is a 45 minute musical first released exclusively on the internet. It stars Neil Patrick Harris as Dr. Horrible himself, Nathan Fillion as Captain Hammer, and Felicia Day as the love interest Penny.
Dr. Horrible is an aspiring villain with hopes of becoming part of the Evil League of Evil. His arch-nemesis is Captain Hammer is an egotistical super hero who always foils his plans. Dr. Horrible's love interest, Penny begins dating Captain Hammer when he mistakenly saves her. Dr. Horrible then gets a letter from the Evil League of Evil saying he must kill a person to be accepted into the League. He plans to kill Captain Hammer and hopefully win Penny's heart.