Bill introduces the ancestor of the modern clock: the sundial. He illustrates how it uses shadows to indicate the time of day and explains how it influenced the clock. A time lapse then shows how the sundial and clock are in sync as the hours go by during the day.
Mr. Z is trying to take all of the zeros away with him. Jackie, Matt, Inez, and Digit discover that zero has meaning just like any other number. They use real-world examples, such as telling time or taking an elevator to the 10th floor, to show what will not be possible without zero. On their search for Mr. Z, they note that the cars on a highway are traveling very slowly because the zero from the original speed limit, 60, disappeared.
Doc Brown shows Marty his time machine in the DeLorean at the Twin Pines Mall parking lot. Doc tells Marty about the date he invented time travel. He states that the Flux Capacitor is what makes time travel possible.