Helena (Stephanie Leonidas) is a young girl who dreams of running away from her family owned circus to join real life. One night, after telling her mother that she wanted her to die, Helena's mother (Gina McKee) falls ill and needs to be taken to the hospital. While there, the bills pay up, and the circus is in danger of folding. One night, Helena falls asleep and wakes up in a mystical world that is divided in half into light and dark. With the help of Valentine (Jason Barry), her guide, she discovers that the Queen of Light (McKee again) has been placed in an enchanted sleep and can only be revived with the use of a charm, the Mirrormask. During their travels, the pair encounter an unstoppable shadow, which is slowly eating the world. Eventually, they make their way to a small house on the border between the land of light and shadow. The house has a pillar full of hundreds of locks, one of which will fit a key they picked up in their travels. However, before she can try the locks, Valentine betrays Helena to the Queen of Shadows (Yet AGAIN McKee), who has lost her daughter and hypnotizes Helena into believing she is the missing princess.
By the way: The missing Princess (Also played by Leonidas) has escaped to the real world through the use of the MirrorMask which she stole from the Queen of Light, and is enjoying her time in our world by being a delinquent minor, smoking, fighting with her father, and making out with unscrupulous boys.
Eventually, Valentine changes his mind and returns to the room of locks. He finds the right one, and discovers a letter from the runaway princess, announcing her theft of the MirrorMask. He returns to Helena and helps revive her. Helena remembers her mother's advice (Usually when you are looking for something, it is right in front of your face) puts her face into a mirror and finds the MirrorMask.
In the real world, the Dark Princess is destroying all of Helena's artwork, which is destroying the fantasy world (The mysterious "Shadows")
The only way to returns to the real world in through the windows of the fantasy world, so Helena and Valentine race around trying to find one. At last, the Dark Princess believes that she has destroyed all the windows, but misses one that Helena drew on the back of a door to the roof of the apartment. Helena returns to the real world through it.
Helena's mother recovers after surgery, and the circus begins to regain its sales. One night, as Helena helps out, she meets this world's incarnation of Valentine, and asks him to join the circus.