At the end of Season 2 Sam was killed by the demon Jake, but resurrected after Dean made a deal with the demon at the crossroads: his soul for Sam's life.
However, the demon told Dean that she would only give Sam one more year to live, and if he tried in any way to prevent his death, Sam would drop dead on the spot.
When Dean returned he found his brother alive and well and unaware of all that had happened. After everything Dean went through, knowing that his father had died for him, he wanted to protect Sam and decided not to tell his brother that Jake had killed him.
Ellen, who was away when the roadhouse burned down, found Bobby, Dean, and Sam and showed them a map that Ash had prepared just before his death. They discovered that an elaborate gate was constructed by Samuel Colt to bar an entrance from hell.
The demon told Jake that he must open the gate and that the Colt was the key. The whole group arrived to stop Jake, who was surprised to see Sam, insisting that he had killed him. Jake got the gate open but was shot to death by Sam.
The yellow-eyed demon returned and taunted Dean with the possibility that the Sam that Dean had resurrected may not be "one hundred percent pure Sam." He then attempted to kill Dean.
However, John Winchester, who escaped from hell, managed to save his son. Dean shot and killed the demon with the last remaining bullet in the Colt, and John's soul departed, no longer condemned to hell.
Bobby and Ellen were able to close the gate, but not before hundreds of souls and demons escaped. Sam was suspicious and wanted to know why Jake insisted that he had killed him. Dean was forced to reveal the deal he had made at the crossroads, and Sam, upset over the sacrifice Dean had made, promised to save him.
The first episode os Season 3 introduces one new character – a young blonde woman named Ruby, who uses a demon-killing dagger.
Ruby is possessed by a demon that shows particular interest in Sam.
At her urgings, Sam discovers that all of his mother's friends and contacts have been steadily killed off over the years by the yellow-eyed demon.
Ruby won't discuss what it is she wants, but says that not all demons are the same or have the same motivations...