If you were looking for the previously announced Gilbert and Sullivan’s H.M.S. Pinafore in this slot on the ACT 1 theater schedule, you might be disappointed. But instead, you might consider it an opportunity to experience something truly new and different. Indeed, this unusual musical creation, originally produced for the Internet and cannily designed to get around the technical hassles caused by the 2007-2008 Writers Guild of America strike, has attained cult status: a wacked-out contemporary-lite opera gone viral. Screenwriter/director Joss Whedon — whose credits include Buffy the Vampire Slayer, Toy Story, Roseanne and a lot of near-miss TV programs and films — collaborated with family members Zack Whedon, Jed Whedon and Jed’s wife, actress Maurissa Tancharoen, to launch this musical tragicomedy, which is set in Los Angeles and tells the tale of aspiring supervillain Dr. Horrible, his nemesis Captain Hammer and Penny, their shared love interest. The original production rather famously featured Neil Patrick Harris in the title role (excerpts readily available on YouTube). Anne-Geri’ Fann adapts the screenplay and directs the Nashville premiere, and her strong cast includes Patrick Kramer, Daniel Vincent and Lindsay Terrizzi Hess, plus seven others. Look for an over-the-top story approach, plus a strangely catchy pop score that sometimes recalls the musical quirkiness of Pee-wee’s Playhouse. The results definitely bear watching.