Don and Dr. Smith try to cap a volcano that threatens the Jupiter 2: if it stays, lava will overrun it, and if it takes off, the explosive and corrosive gases from the fuming stack will damage it, perhaps severely! But the territory around the volcano harbors primitives who worship the great Protineus
, a machine they claim will evolve them. Don and Smith have contaminated and damaged this, so Protineus
decrees that they must die!