The American hunters had finally defeated the British Men of Letters. Okay, maybe not exactly "defeated", but still, they were pretty satisfied with the result of their mission. Their American base of operations had been destroyed, their leaders killed. It was going to take a while before they ever decided to go back to America again. They were happy, they could finally celebrate. At that point, all Sam and Dean Winchester needed to do was find Cas and Kelly Kline before the birth of baby Satan and extract the baby's grace before he was born. They even had their mother back to normal. Those were the thoughts on Dean's mind when he saw his brother come back to the bunker alive.
They needed a break, but they weren't going to get one. That was what became pretty clear the moment Sam showed the picture of Lucifer that he'd recovered from the British Men of Letters. Satan in person was alive, and he was looking for his son, who was going to be born soon.
"This blows" said Dean, while he was riding in the Impala with his mother and brother, going towards the place they found out Castiel was hiding Kelly Kline.
Sam did't say anything. He didn't want to say anything, actually. Since he first found out that he was supposed to be Lucifer's vessel, he always thought he was supposed to the the one to take care of the archangel. He was, however, never able to do it. One way or another, Lucifer always managed to get away free. And Sam, who had been possessed by him, and tortured by him in Hell, hated that idea more than anyone else. All he wanted was to stop him, once and for all.
That was the reason why, while they were sitting in the car, the only thought on his mind was Lucifer, how to face him, since he was going to show up for sure at some point. Dean's plan was to leave before the Devil could get there, but Sam knew it wasn't going to be an easy task to accomplish. Lucifer, after all, was incredibly resourceful and he had the really bad habit of showing up when it was least needed. Therefore they needed to be prepared to face him, to defeat him. Because there was nothing Sam wanted more than to kill the Devil for good.