Meg was a girl possessed by a demon, they banished the demon and Meg died.
They found their dad and rescued him, only for Dean to realise that he was possessed by the main demon (the one that had killed their mum and they were tracking). The dad/demon held Sam 'hostage' (unable to move) and started killing Dean. Then John (the dad - just remembered his name) managed to overpower the demon temporarily and got control of his body back and stopped killing Dean.
They managed to get the demon out of John, then they were driving down a road - Same driving, John in the passenger seat, and Dean laid across the back. When a truck hit the car. The truck driver got out and we see that he is possessed. The last shot was the camera zooming out as we see the car as a whole, and all the guys in really bad shape.