Season 1

Season 1 starts with the pilot episode that lays the foundation for the story. It starts with a flashback to 22 years ago, to the Winchester home. Sam and Dean's mother (Mary) is attending to a crying Sam, age 2 at the time, in his crib. A demon appears in the doorway (we later learn it to be the "yellow eyed demon"), places Mary on the roof and slices open her midsection. The father (John) rushes into the room to see what is going on. He tells Dean, age 4 at this time, to take his little brother out of the house. John attempts to save his wife to no avail, as the room bursts into flames. The show flashes ahead 22 years and Dean seeks Sam's help because John has gone on a "hunting" trip and has yet to return. Sam is currently living with his girlfriend, but reluctantly agree to search for him. They realize that John is searching for the "yellow eyed demon" and Dean asks for Sam's help to find him. Sam refuses because he wants to live a normal life, however, when they return to Sam's home they find his girlfriend in the same position as his mother exactly 22 years ago. After the death of his girlfriend Sam agrees to help Dean find their father. Season 1 continues with Dean and Sam hunting the supernatural while looking for their father who they find at the end of the season.

Yellow eyed demon - http://images.buddytv.com/battleimages/b2001786/100383297/95820ed3-b03b-4557-a48e-0d6d6ea7edd8fl2.jpg

On to Season 2