Maxwell's Key in the Mirror is a magical tale about a young boy who turns ten years old. While he is visiting his great-grandparents, he takes a tumble reaching for a key that fell off a mirror. The fall renders him unconscious, and when he wakes up, he has been transported to a new world where he is on his own.
A girl named Penelope, living in an underground house in the desert, appears and believes Maxwell can help her save her world from the mandatory separation of people caused by the fruit of the walking trees. In return, she would help him find his way home. Penelope tasks Maxwell with following a map to find her friend Doren, who can help.
Maxwell and his newfound friends fight the walking trees together, but their plan puts Maxwell in jeopardy. After burning down the trees, Maxwell thinks all is well, and he will now be headed home. A case of mistaken identity inadvertently lands Maxwell in a situation where he'll have to continue his quest to get back to his own world.