Penny discovers an egg on the transportation machine left behind by the Taurons. After Dr. Smith makes off with it, Penny is trapped in a maser beam accidentally activated by Debbie the bloop. Both vanish. Will wonders if the machine has had anything to do with her unexplained disappearance and, with the Robot's help, effectively brings her back. She has no memory of the experience, and everyone comes to take Will's report as just fanciful imagination. Nevertheless, Prof. Robinson thinks they should destroy the machine to keep Will from playing around with it. Distressed by its imminent and needless destruction, Will takes the Robot and has himself beamed to Earth, with instructions for the Robot to bring him back in four hours. On Earth, Will is taken for a runaway with fanciful tales; back on the planet, Dr. Smith takes the Robot away for selective reprogramming while John and Don arrive to destroy the alien machine.
—statmanjeff