Main article: Supernatural (season 1)
Season One consists of 22 episodes that aired in the United States beginning September 13, 2005, and ending May 4, 2006. The first sixteen episodes aired on Tuesdays at 9:00 pm, after which the series was rescheduled to Thursdays.
Two brothers, Sam and Dean Winchester, team up to find their father, John, who went missing on a hunting trip. However, their father isn't a typical hunter: he hunts supernatural creatures like ghosts, vampires, and various demons and he's trained his sons to do the same. Along the way, Sam and Dean save innocent people, fight creatures and ghosts, and collect clues to their father's whereabouts. Sam begins to mysteriously develop psychic abilities and visions as they travel. They eventually find their father, who reveals that the creature that killed Sam and Dean's mother years earlier is a yellow-eyed demon and the only thing that can kill him is a legendary gun created by Samuel Colt.
Main article: Supernatural (season 2)
Season Two consists of 22 episodes that aired on Thursdays at 9:00 pm in the United States beginning September 28, 2006, and ending May 17, 2007.
The season follows Sam and Dean as they deal with their father's death and continue to hunt the Yellow-Eyed Demon with assistance from new allies Ellen, Jo, and Ash. Part of the Demon's master plan is eventually revealed as he gathers Sam and others like him, leading to Sam dying. Dean makes a deal with a crossroads demon to bring back Sam in exchange for his soul, which will be collected in one year. In a final confrontation, in which Dean kills the Yellow-Eyed Demon, a Hell's Gate is opened, releasing numerous demons from within.
 Season Three
Main article: Supernatural (season 3)
Season Three consists of 16 episodes that aired on Thursdays at 9:00 pm in the United States beginning October 4, 2007, and ending May 15, 2008. Originally 22 episodes were ordered for the third season, but production was halted on December 5, 2007, upon completion of the twelfth episode due to the 2007–2008 Writers Guild of America strike. The season number was shortened to sixteen episodes, with four new episodes airing in April and May 2008.
The season mainly focuses on trying to save Dean from his deal and tracking down the demons that were released from the gate. Along the way, the brothers meet a demon named Ruby, who has an interest in Sam and claims to be able to help save Dean. Also, they meet Bela Talbot, an "acquirer" and seller of occult objects, who is constantly a thorn in their side. The brothers eventually learn what demon holds Dean's contract; a being of great power named Lilith. The brothers, along with Ruby, track her down and attempt to kill her. Lilith is unable to stop Sam due to his mysterious abilities, however, Dean's contract expires and his soul is taken to Hell.
 Season Four
Main article: Supernatural (season 4)
Season Four consists of 22 episodes that aired on Thursdays at 9:00 pm in the United States beginning September 18, 2008, and ending May 14, 2009.
Dean is rescued from Hell and brought back by an angel named Castiel; one of several angels who appears throughout the season. The rest of the season follows the brothers as they work with Castiel to stop Lilith's plan of breaking the 66 seals, which would allow Lucifer to walk free once again. Sam and Dean's relationship is also strained as Sam starts siding with Ruby over Dean. He also begins to give into his demonic side by drinking demon blood to become strong enough to defeat Lilith. After tracking down and killing Lilith, Sam learns her death is actually the last seal and Ruby was deceiving him all along. Dean arrives too late to stop him, but kills Ruby as Lucifer's prison opens up beneath them.
 Season Five
Main article: Supernatural (season 5)
Season Five consists of 22 episodes and was rumored to be the last season, due to Eric Kripke's statements over the years that he had planned for the show to run for only five seasons. Despite this, Jared Padalecki and Jensen Ackles had contracts for a sixth season, and The CW renewed the series on February 16, 2010.
The fifth season revolves around the fight to stop Lucifer and save the world from the Apocalypse. Throughout the season, Dean and Sam battle both angels and demons as they fight their destiny to become the vessels of Michael and Lucifer. Unable to defeat Lucifer, they collect the rings of the Four Horsemen of the Apocalypse, which act as the keys to Lucifer's prison. The idea of destiny versus freedom and choice play a big role. Throughout the season, Dean, Sam, Castiel, and Bobby each have a crisis as they near giving up. However, through each other's support, they continue forward until the end.
 Season Six
Main article: Supernatural (season 6)
Creator Eric Kripke originally planned for the show to last only five seasons, but due to increased ratings from the fourth and fifth seasons, the CW network renewed the series for a sixth season. Kripke has confirmed he will not be returning as showrunner; however, he will still remain as a hands-on executive producer, leaving Executive Producer Sera Gamble to take over the reins.
Gamble said the sixth season of the show will focus on the brothers' relationship. The season will steer towards the format of the early seasons with "lots of meat-and-potatoes closed-ended episodes, and...a season-long story arc to weave in". According to a press release by the CW, Heaven and Hell are now in "complete disarray", forcing Sam and Dean to reunite to "beat back the rising tide of creatures and demon-spawn" that "roam across a lawless and chaotic landscape".
Filming for the season began with the Ackles-directed fourth episode, "Weekend at Bobby's", to give the actor enough time for preproduction. Misha Collins will return as series regular Castiel, as will Jim Beaver as Bobby Singer. Mitch Pileggi will make multiple appearances as Sam and Dean's grandfather. The recurring role of Christian, the maternal cousin of Sam and Dean, was given to Corin Nemec; he is described as "capable, calm under pressure, and very good at his job". Kim Rhodes will return as Sheriff Jody Mills, as will Fredric Lehne as Azazel.
The season airs on Friday at 9:00 pm in the United States on the CW and premiered on September 24, 2010.