Marty: "I don't know, maybe we could just take Clara with us."
Doc: "To the future? As you reminded me, Marty, I'm a scientist, so I must be scientific about this. I cautioned you about disrupting the continuum for your own personal benefit. Therefore, I must do no less. We shall proceed as planned, and as soon as we return to 1985, we'll destroy this infernal machine. Traveling through time has become much too painful."