The series starts with two brothers, Sam and Dean, whose mother died in a fire when they were kids, on the road trying to find their father, a ghost/demon hunter who went missing. While on the road, they started saving innocent people by battling ghosts, demons, vampires, and other creatures following a "Monster of the Week" theme.
After they found their father, it was revealed to them that the creature that killed their mother was a demon named Azazel, and they were told how to kill that creature. The boys later go hunting for Azazel, whom they end up killing in the second season. That's when the plot starts thickening up. Dean makes a deal with a demon to save Sam, and as part of the deal, he has only one year left on Earth before he is sent to Hell.
One of the reasons that was able to run for 15 seasons was its ability to evolve. While the series might have started off as a simple horror series, it quickly evolved and found its unique voice by embracing some of the more humorous elements.
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