Immediately after Sam frees Lucifer from Hell, he and Dean become targets of both Heaven and Hell. They are threats because they are destined to be the respective vessels of Lucifer and the Archangel Michael, the eldest of the archangels and the one responsible to casting Lucifer from Heaven, who will fight in a destined match to determine Earth's future. This would result in the deaths of millions of innocent people who would be in the crosshairs. If Dean refuses Michael, Lucifer will destroy the human race with the Four Horsemen, the living embodiments of the four plagues, however Sam saying yes to Lucifer will only speed up the devil's plans. Additionally Bobby is crippled and wheelchair-bound. Faced with no other choice, Sam and Dean team up with their angel ally and friend Castiel to look for a means to defeat both Heaven and Hell whilst battling angels, demons and even archangels.
