The film opens with Professor Mangetsu, a Japanese astrophysicist and his daughter, Miyoko, being informed and startled about the approach of a giant black hole that previously destroyed the space probe Voyager 5.

Frustrated by the multitude of problems at school and at home, Nobita wonders if his life would be easier if magic really existed.

Upon returning, Nobita tells him that they can use the "What if" Telephone Booth to enter a world where magic is normalized.

They fly there to examine the crash site, where they meet Miyoko, who is a high-class magic user and states that a demonic planet will destroy Earth very soon.

As more natural disasters occur, Doraemon's group decide to help "Miyoko" to finding the tome to defeat the demons.

At the same time, they find the real Miyoko who appears as a mouse, whose curse is lifted under the light of the moon.

As Doraemon is afraid of mice, the group casts a different spell to turn Miyoko into a cat, as their power cannot lift the curse.

A fake ending plays but then Nobita realizes that the world will still be threatened even when they go back to normal.

The Demon King, enraged by the actions of Doraemon's group, brings his whole army to invade Earth at once.

In the real world, Miyoko, her father and the space center are relieved to see that the black hole vanished before reaching Earth.

After the credits, Nobita tries to cast a spell, he surprisingly gets the same result as in the Magic World but attributes it to the wind while the broom they used to escape Medusa still sits on the tree.