A kid finds giant robot parts falling from the sky and decides the logical next step is to build a weapon in a mirror world, then immediately regrets it when an alien girl shows up asking to borrow it for an invasion. The entire movie is Nobita learning that some secrets stay secrets for a reason, but not before he's accidentally responsible for starting the very thing he was trying to prevent. Lilulu becomes the moral center of a story that pretends to be about stopping an invasion, when it's really about whether a kid can trust someone from the other side.

Pieces of a giant robot fall from the sky, and Nobita and Doraemon carry them into the mirror world, where they put the machine together and name it Zandacross. Realizing it is a dangerous weapon, they keep it secret. Then a mysterious girl named Lilulu turns up and asks for Zandacross to use in an invasion, though she herself does not agree with attacking Earth. As a robot army moves against the planet, Doraemon and his friends team up to stop it, only to find that Lilulu may be the one person who can halt the invasion. The film is an adventure mixing comedy and fantasy.