The Demonic War Continues.

Picking up at the end of season 4, after Sam unwittingly released Lucifer by killing the Demon Lillith, the brothers are miraculously transported aboard a plane from which they witness the beginning of the apocalypse. As Lucifer begins to bring about the end of the earth the brothers team up with the angel Castiel in order to track down the only ally that can help them now--God.