[Marty and Doc have just arrived back in 1955]
Doc: Sometime today, old Biff will show up to give young Biff the Almanac. Above all, you must not interfere with that event. We must let Old Biff believe he succeeded, so that he'll leave 1955 and bring the DeLorean back to the future.
Marty McFly: Right.
Doc: Once Old Biff is gone, grab the Almanac anyway that you can. Remember, both of our futures depend on this.
Marty McFly: You don't have to remind me of that, Doc.