From the back of the box:
Crusty Demons combines the freeform tricking, high-octane stunt mayhem and bone breaking crashes in this free roaming world. It breaks new ground by rewarding players to engineer spectacular crashes and inflict massive injuries and pain to their rider.
The infamous Crusty Demons are a group of extreme motocross athletes and the stars of the Crusty Demons of Dirt movies.
During an attempt to perform a world-record breaking 5-rider triple back flip in front of an adoring crowd, the Crusty Demons are interrupted mid-flight by the Devil, who informs them that unfortunately they are all about to die.
Satan offers the Crusty's the chance to become immortal giving them the ability to crash the most spectacular, bone breaking crashes and get up unharmed to ride again. The price for this immortality is the Crusty's souls for all eternity. Seemingly with no choice, the Crusty's sign away their souls.
The Devil offers them a chance to win their souls back by playing the damnation game. The Crusty's must travel the world in a tour of destruction where anything goes, completing the Devil's Checklist at each world location in a bid to entertain the Devil.
Completion of the checklist allows the Crusty's to race the Devil himself, and only beating Satan at his own game will allow the Crusty Demons to regain their souls and avoid eternal damnation.