A female alien meets the Robinsons and uses them to help erect an arch to bring her people to colonize the Robinsons' planet.

While John and Maureen are away observing volcanic activity, the alien scientist Lemnoc captures Don, Judy, Penny, and Dr. Smith and replaces them with android duplicates. Lemnoc then forces Will and the Robot to train the duplicates in survival skills, hoping that they will be an inspiration to his own people, who are losing the will to live.