Key is no ordinary imaginary friend. He's the only one he knows of who's actually come back from the sleep that all imaginary friends succumb to when their creators forget about them. Did the girl bring him back or did he wake up on his own? Does she need his help? Where is she? These are all questions that help him determine that he needs to find her.
There are a few problems though. For one, he can't remember his past. Images and moments come and go but they are all so fuzzy. The next problem is, that shorty after he wakes in the Wylds, (a world between worlds filled with everything everyone has ever imagined or dreamed about) the evil witch Baba Yaga becomes obsessed with capturing him and using the immense power he possesses for her own ends.
His travels between the Wylds and Clockwork (what those from the Wylds call our world) gain him a small group of friends, namely, Tersius; a man dressed in red and able to wield three swords at once, a Welshire Cat; cowardly and a bit insane, Ashen; the blood thirsty princess of a long collapsed empire, and Dugan; a bad man turned good.
While the Wylds are full of glamour and imagination, they are deadly to anyone who steps off the path, Clockwork is almost equally as dangerous. Their lack of color and imagination threaten to unravel the existence of anyone outside of its realm of reality. And there are men in black suits and fedoras constantly on the look out for people like Key and his friends. Men who believe it is their soul purpose to destroy anything that makes sense. They are called Clockworkers, and mercy is something they do not believe in.
While Key slowly learns bits from his past, seemingly at random he is thrust into communication with the girl that brought him into existence through dreams. While visiting her in one of these dreams he has a horrifying revelation. She is at a special school being trained to become one of these Clockworkers, and she doesn't want him to come to her aid. She likes her life the way it is.
But there is more lurking in the depths of his past that he cannot remember. Something that must be brought to light before either of them can continue on with their lives, and something that if not discovered, will bring them to madness. And there isn't much time.
With his friends, his abilities, and his magic set of chalk, Key sets out to infiltrate a Clockworker compound in order to kidnap the girl who made him live so that he in turn, can help her live.
But with Baba Yaga continually ensnaring him in her traps, and Clockworkers constantly interfering, will he be able to save her in time? Or will he pass into darkness and be lost?