The show begins with Sam and Dean Winchesters' mother being killed by a yellow-eyed demon while he's dripping his blood into baby Sam's mouth. Sam and Dean grow up as hunters, who find and kill the supernatural with their father, John, who's not exactly father-of-the-year. After John goes missing, Sam and Dean try to find him, but Sam must return to college. He finds his girlfriend dead, having been murdered by the yellow-eyed demon once again. The first two seasons are Sam and Dean searching for this demon and their father, only to discover Sam's psychic abilities on the way. They kill many ghosts, demons, and other creatures, and are even hunted by the police in some episodes. Things get hairy when Sam dies and Dean has to make a deal with a demon to return him to life. (As I'm only about 6 episodes into season 4, I'll just stop here and leave a bit of mystery about the rest of season 3.)
Even though it is a show that plays once a week, it does not look at all cheap or fake. It looks vibrantly real (despite being quite a dark-toned show). The jump-scares are frightening and there are copious amounts of blood. All-in-all, this show is captivating and fantastic. The actors are amazing and the chemistry between Jensen and Jared is intense. They almost seem like they could really be brothers. I think anyone with access to this show should watch it, as it is fast-paced and wonderful. You'll probably love it as much as I do.