The Robinsons must land to repair their ship. As the pass, space capsule Alpha-784 latches onto the Jupiter 2 magnetically and follows them in. It contains a disassembled female robot. The Robot helps reassemble her, and even forces the Robinsons to give him a crucial part to re-energize her! But she's evil, and her escape has not gone unnoticed by aliens willing to destroy the Robinsons if they will not help find and recapture the robot. Re-energized, she proves a match for her pursuers - so how will the technologically less capable Robinsons deal with her?