A woman named Molly and her husband are driving down the highway and swerve to miss a farmer who's walking down the road. Their car hits a tree and when Molly wakes up her husband is missing. Sam and Dean discover Molly and find out that the farmer haunts the highway and Molly is most likely his next victim. Sam and Dean set off to stop the farmer before he can claim her.
Dean: Smells like old lady in here. (runs into a corpse of a hung old lady) That would explain why.
Dean: Y'know, just once I'd like to round the corner and see a nice house.
Sam: It's an old country custom, Dean. Planting a tree as a grave-marker.
Dean: You're like the walking encyclopedia of weirdness.
Sam: Yeah, I know.
This was a really powerful episode and amazing acting by Tricia Helfer. The ending really surprised me, I did not suspect that she was actually a ghost.