After visiting 2015, Marty McFly must repeat his visit to 1955 to prevent disastrous changes to 1985...without interfering with his first trip.
 
FUN FACT:  According to Michael J. Fox, he found out there was going to be a sequel to Back to the Future (1985) when he watched the VHS version and the words "To be continued" were added at the end. He immediately called his agent to make sure he was going to be in the sequel.