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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?
The robot forces John to give him a subatomic energizer by trading Will's freedom for the device. John gives him the mechanism, but then he and Don follow the robot. There is a point where the Robot rolls off camera, but his shadow is still clearly visible. It abruptly vanishes, rather than continuing to move as it would if the Robot continued to roll. Clearly, separate shots were patched together without much attention to continuity.