Supernatural follows brothers Sam (Padalecki) and Dean Winchester (Ackles) as they travel the continental United States hunting supernatural creatures that pose a threat to society. At times, they are assisted by fellow "hunter" and family friend Bobby Singer (Jim Beaver). Their greatest enemies come in the form of demons, corrupted human souls that have escaped from Hell. A cloud of black smoke in their true form, they take possession of human hosts.
Twenty-two years earlier than the series' main storyline, the demonic tyrant Azazel fed his blood to Sam and other infants, to imbue them with demonic abilities. He gathers them together in the second-season finale "All Hell Breaks Loose" and coerces the young adults into a fight to the death to determine a leader for his demonic army. Sam is killed by the super-strong Jake Talley, but Dean sells his own soul to a crossroads demon in exchange for Sam's resurrection, a contract that leaves Dean with only a year to live. The Winchesters, Bobby, and fellow hunter Ellen Harvelle kill Jake and Azazel, but are too late to prevent the release of hundreds of demons from Hell.
One of the freed demons is Ruby (Cassidy), a former witch who claims to oppose the demonic world. She frequently helps the brothers throughout the third season with her demon-killing knife and knowledge of witchcraft, but Dean mistrusts her manipulative nature. As the deadline approaches, the Winchesters learn that Azazel's successor, Lilith, holds the contract to Dean's deal. He now only has one day remaining before he is sent to Hell.