The serial is as much about demons, angels, various monsters as it is about the two brothers. Two brothers, Dean and Sam, begin hunting demons in their father’s foot steps. Initially, their mother had died under mysterious circumstances, burnt in flames while being stuck to the ceiling. When Sam’s girl friend meets the same fate, the brothers join hands to hunt.
Their father has been missing. Slowly, we come to know (over 4 seasons, 5th is on in US, will be in India next year) the background story of the Winchester family. They eventually lose their father, but find a "father figure" in fellow hunter Bobby.
There are many folklores and urban legends besides demons, vampires and angels in this serial. Each non-human creature has a specific characteristic.Dean is cocky, loves his 1967 Chevy Impala, fears air travel, drinks beer, watches porno (It IS a family show, so do not worry. I am just mentioning attributes :)) and always looks after his brother. Sam is a reluctant hunter at first, uses internet to search for paranormal phenomena and "jobs" and takes many risks. They regularly pose as officers (law, medical or anything) with fake badges to investigate their case.
As is to be expected, they do get into trouble with law sometimes. There are elements within the "Supernatural" world like the Colt, which can kill Demons with special bullets, spells which are cast to "arrest" demons.Ignoring SuperNatural as "just" a horror, creepy show with special effects to my mind would be a mistake. It is as I said about brothers, their trials and tribulations.
While almost each episode was stand-alone in the first season, it has a much cohesive, connecting plot for the whole season from the 2nd season. By the 4th season, it has a story running through out with some unlinked episodes in between. Special effects are very cool indeed. Overall, this is great fun to watch, especially if you are a fan of Horror genre.