Dr. Smith causes another accident, and once again the Professor banishes him from the camp. While spending the night in the wilderness, Smith is accosted by dozens of tiny robots who hold him prisoner. The mechanical men then approach the Robinsons and demand their Robot come to them to be their great leader. The Robot complies, but the sinister mechanical men find him too kind and tolerant for their needs. Preferring Smith's treachery and ruthlessness, they transfer Smith's personality to the Robot's body and vice versa. Now Smith-minded Robot commands an unstoppable mechanical army on a mission to seize the Jupiter 2 from the Robinsons, while the Robot-minded Dr. Smith feels he is obligated to stop them himself.