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While trying to cap a volcano, Don (Mark Goddard) and Dr. Smith (Jonathan Harris) are captured by a band of primeval creatures, who take their orders from a computer named Protinus. As the rest of the Robinson party conduct a search-and-rescue mission, Don and Smith are faced with the Hobson's Choice of either being executed on the spot or dying a slow death as the lava consumes them. In the course of events, Smith saves Don's life, and for one brief shining moment the two longtime antagonists form a warm (if not lasting!) friendship.