Sammy is still hallucinating from his time in hell and the devil follows him around trying to bait him into doing something stupid.
Castiel or the leviathans that have now taken over his vessel walk into a dam and explode releasing them all into the water supply infecting anyone that comes into contact. After all that time they are hungry and go on a feeding frenzy in which the brothers and Bobby attempt to stop but they realise their weapons that had been so effective on other Hell Creatures dont work.
Love the scene where the surgeon / leviathan takes a patient out in the middle of the night for "surgery" then promptly eats them.
Still one of the best shows on Television.