Giant robot parts crash from the sky, prompting Nobita and Doraemon to transport them into a mirror world where they assemble them into a colossal machine called the Zandacross. Recognizing its destructive potential, they keep the new weapon hidden. A mysterious girl named Lilulu arrives and requests the Zandacross to aid in an invasion, though she refuses to attack Earth. Doraemon and his friends unite to prevent the robot army from launching a siege, yet it becomes clear that only Lilulu holds the key to stopping the impending threat.