If you're a fan of Terry Pratchett's long-running, multi-stranded, and wholly silly Discworld series of novels, you're either about to get incredibly excited or incredibly disappointed. BBC America just released its first trailer for The Watch, the upcoming fantasy-comedy TV series based on Pratchett's work. And it all looks like it's taking some big ol' swings. Here's the official synopsis for the series, provided by BBC America: The Watch follows an unlikely group of misfits, The City Watch, who are forced to find the guts to save the world, surprising even …