Crowley offers the brothers a deal: an alpha in exchange for Sam's soul. He then sends the brothers to investigate a murder that appears to be a werewolf attack in Buffalo, New York. The two brothers arrive at a scene of another murder and learn that the second victim was murdered in the same way as the first. After some research, Sam discovers Cal Garrigan is the common denominator between the killings. They interview Cal and meet his girlfriend Mandy, Mandy's son Aiden and their dog Lucky. The brothers spy on Cal overnight, but see nothing; after they leave, Cal is killed by Lucky. Sam sees the dog Lucky transform into a man and follows him out of the house. There is a chase and Lucky is hit by a van. It turns out the killers are not werewolves but skin-walkers. Sam and Dean trap Lucky at an animal hospital and convince him that it would be better to give up his pack than to risk the family he loves. Lucky tells the two that the skin-walkers plan to infiltrate families as pet dogs and upon given a signal, turn the families into skin-walkers. Dean decides to eliminate the leader of the pack and there is a shootout. All the skin-walker pack except Lucky are killed, but he escapes, wounded. Lucky then goes back to Mandy to thank her for her kindness, but she tells him to leave and never come back. Afterwards, Sam, who Dean has been insisting is no longer the same without his soul, finally admits Dean is right and that he doesn't care about Dean or anything and asks Dean's help to get his soul back.
Source: Wikipedia
Source: Wikipedia