The Crooked Man first introduces himself to David following his mother's death, periodically appearing to him in his panic attacks and nightmares. Once, he is seen looking down at David from his bedroom window, though investigating the room reveals nothing but a magpie. In his dreams, the Crooked Man proclaims David as a future king and reminds him that "we are waiting."
Throughout this time, David is in emotional turmoil: quite apart from the death of his mother, his father has remarried a woman named Rosie; David immediately resents her - even more so when she gives birth to his half-brother Georgie. The Crooked Man exploits this growing tension: following a bitter argument between step-mother and step-son, he is able to lure David outside with the sound of his mother's voice. When an out-of-control German bomber crashlands in the garden, David is forced to hide in a crack in a wall - unexpectedly transporting him into the fairy-tale kingdom where the Crooked Man resides.
Though David is able to find shelter from the dangers of the forest, along with a protector in the form of the Woodsman, the Crooked Man erases any sign of the portal leading back to the real world, ensuring that David remains trapped. Having been informed that the King and his eponymous Book of Lost Things might be able to help him, David sets out towards the castle - unaware that he's actually playing into the Crooked Man's hands. The trickster spends a great deal of time ensuring that his target continues the journey, not only watching him almost every hour but even killing the scouts sent after him by the Loup horde infesting the land.
After being separated from the Woodsman, David eventually teams up with another ally - a knight-errant by the name of Roland. During this time, the Crooked Man first introduces himself to David, taunting him with the knowledge that nobody misses him, that his father has forgotten about him just as he forget about David's mother: he even shows him images of his father, Rose and Georgie dancing together - before then showing him his father making love to Rose. David is so enraged by this, that he actually tries to kill the Crooked Man, slashing him with his sword in a temper. Unharmed, the Crooked Man departs - but not before he explains that he can help David find his mother, now a prisoner in this world, and get them both home: all he asks in return is something "so small you won't even miss it."
Following this encounter, the Crooked Man doesn't show himself to David for some time. After he and Roland eventually succeed in defeating the Beast, however, David wanders away from their brief encampment - and is pulled underground by the Crooked Man. There, he gleefully undermines the friendship between the two travelling companions, revealing Roland's homosexuality - and implying that Roland secretly lusts after David. He also makes the bargain he promised to make, insisting that all David has to do in order to be reunited with his mother and sent home is to give the Crooked Man a name - specifically, Georgie's name. However, Roland is able to rescue David before he can consider the deal.
After the two friends gradually learn to trust each other again, they find themselves at the Fortress of Thorns, where Roland's long-lost lover, Raphael was last seen. Roland enters, but when he fails to leave, David follows - much to the anger of the Crooked Man, who arrives too late to stop him. Though he finds Roland and Raphael dead, David is able to outwit the monster that dwells in the Fortress and escape; once again under the Crooked Man's protection, he makes it past the Loups pursuing him and into the safety of the King's castle.
Though the King is welcoming enough to provide David with a room, he isn't as helpful as he'd hoped. Later that night, David sneaks out of his room and eavesdrops on a conversation between the King and the Crooked Man: the King is heard mentioning a mistake he made as a child, and the Book of Lost Things is dismissed as valueless to everyone except the King himself. Examining the book shortly afterwards, David finds that it's actually the King's childhood scrapbook - and that the King himself is really Rose's great uncle, Jonathan Tulvey, who had disappeared from the real world as a child.
Investigating further, David discovers the soul of a little girl named Anna imprisoned in a glass jar. She explains that she was actually Jonathan's little sister, sacrificed to the Crooked Man - who ate her heart before her eyes. It is soon revealed that the Crooked Man has, in fact, been selecting the kings and queens of this land for untold centuries, taking dissatisfied children from the real world and offering them the throne; all he asks in return is a younger relative the future monarch didn't like - usually after encouraging him or her to give into their feelings of resentment and jealousy. While the new monarchs are trapped in a world infested by creatures from their nightmares and doomed to live out the rest of their lives trying futilely to atone for their crimes, the Crooked Man gets to devour the heart and life of the sacrificed children, prolonging his life with the years they would have lived.
Now, David has been selected as the future king, and Georgie is to be the Crooked Man's next victim.
Outside the castle, the Crooked Man has almost spent the years he took from Anna: needing to speed up David's choice, he leads the Loup army into the castle by means of a secret tunnel, hoping to use them as leverage to force David to agree. As the Loups overwhelm the castle's defenders and the Crooked Man's final minutes tick by, he tries one last time to get David to give up Georgie, once again demanding the name of his half brother or die at the hands of the invaders. At first, David appears to relent - but at the very moment the Loup leader kills the King, he replies "his name is 'brother'."
Enraged at seeing his last chance at renewing his life slip through his fingers, the Crooked Man literally tears himself apart, revealing a swarm of live insects and other vermin. Immediately dispersed, the insects scurry away in all directions - each one of them dying as Anna's stolen life fades away and the Crooked Man finally expires.