Sulking over a friend's fancier toy robot, Nobita Nobi ends up at the North Pole, where he digs a heavy mechanical orb out of the snow. He and Doraemon set it loose in a mirror world, and the robot parts it calls down from the sky are assembled into a giant machine Nobita names Zanda Cross. The android girl Riruru arrives to reclaim it, and Nobita, Doraemon, Shizuka Minamoto, Suneo Honekawa, and Takeshi Gouda learn that the machine is a weapon meant to spearhead a mechanical army's invasion of Earth.