Reset the 1955 timeline before you disappear and lightning strikes the clock tower.
"Wait a minute, Doc, are you telling me you built a time machine...out of a DeLorean?"
The photo of the McFly family is slowly fading... It's 1955, and you're wrapped up in a time paradox with Biff, Lorraine, George, and Doc Brown! Cooperate to move around Hill Valley to get the DeLorean ready, avoid Biff and his gang, help George and Lorraine fall in love, and crank the DeLorean up to 88 MPH — all just in time for the lightning to strike the Clock Tower, sending you back to the future!
In the fully co-operative game Back to the Future: Back in Time, each player takes on the role of a major character from the movie: Marty McFly, Doc Brown, Jennifer Parker, or Einstein the dog. The objective of the game is to have the characters move around 1955 Hill Valley, collecting certain items in an effort to fix Doc's famous DeLorean time machine, defeat Biff Tannen and his gang of trouble-making friends, while ensuring that Marty's parents fall in love. Only when that is accomplished can players then accelerate the DeLoreon to 88 MPH down Main Street before the clock tower strikes 10:04 pm!
—description from the publisher