By Neil Gaiman and Dave McKean
William Morrow

The Jim Henson Company approached Messrs. Gaiman and McKean, inquiring whether they would be interested in making a fantasy film. They have collaborated on a number of projects over the course of almost twenty years, their most noteworthy achievement being the award-winning Sandman series. Even though The Jim Henson Company only had a $4 million budget, Gaiman and McKean were intrigued by the offer and agreed.

In the book's introduction Gaiman details how he and McKean worked together to create the film's story and screenplay, which was a tad difficult because of their different approaches to writing. McKean outlines an entire project on cards, aware of every aspect and idea, before writing a screenplay while Gaiman talks until he's ready to write and then allows a screenplay to flow out of him as he works.

Mirrormask is about a young girl named Helena, whose parents run a traveling circus. She is tired of her life and wants to get out of it. Her mother falls very ill and is hospitalized, so Helena stays with her aunt. Helena loves drawing and covers the walls of small bedroom she is staying in with her pictures. One night, she has an odd dream and then unknowingly walks into a city on the other side of reality.

It's a magical world that is divided into the White City and the Dark Forest. Helena is mistaken for the Princess, who stole a charm that caused the White Queen to fall into a sleep she can't be awoken from. Her slumber has broken this world's balance and shadowy tendrils seep out of the Dark Forest, destroying everything in the White City they touch. Helena volunteers to find the charm because she has seen herself sleeping back in her aunt's flat and assumes she is dreaming.

As the adventure progresses, Helena learns that she may not be in a dream after all. She becomes aware that she has switched places with the Princess, who ran away from her mother, the Dark Queen. Helena discovers even graver news when she realizes that this strange, new world she is trapped in are her drawings pasted on her aunt's bedroom walls. When the Princess sees Helena in the drawings, she begins tearing them down, destroying the world she ran away from.