As the Jupiter 2 travels through space, Doctor Smith is seen caring for a plant he took from the last planet they visited. Will warns him to throw it overboard, but before he can, the Jupiter 2 passes through a radiation belt, causing the plant to mutate and grow. Major West "recruits" Dr. Smith to accompany him in the Space Pod to the planet's surface in order to locate and disable the source of the attack. There they find the vestiges of an ancient warrior society called the Sobrams. The last living Sobram forces West to agree to remain behind and maintain his military complex.