Inside of his book, adventurous Harold can make anything come to life simply by drawing it. After he grows up and draws himself off the book's pages and into the physical world, Harold finds he has a lot to learn about real life.

Inside of his book, adventurous Harold (Zachary Levi) can make anything come to life simply by drawing it. After he grows up and draws himself off the book's pages and into the physical world, Harold finds he has a lot to learn about real life - and that his trusty purple crayon may set off more hilarious hijinks than he thought possible. When the power of unlimited imagination falls into the wrong hands, it will take all of Harold and his friends' creativity to save both the real world and his own.

A young boy lives inside of a comic book and he can make anything come to life, such as his friends Moose and Porcupine.

In the modern day, Harold is all grown up and refers to the narrator as "old man". One fateful day, the narrator goes missing and Harold uses his trusty purple crayon to draw a door into the real world, in search of him, with Moose joining him, Porcupine being left behind, before getting lost in the real world.

Harold and Moose go on to ride a bike that Harold had drawn, in search of the old man. Harold believes he is actually his father. Terri, The mother of an imaginative boy named Mel accidentally hits the bike with her car, getting a puncture in the process, to which Harold draws a new one, much to Terris confusion as she hadn't seen him holding his crayon. Mel manages to convince Terri to let the duo stay in a room just above her garage. Later that evening, Mel invites Harold downstairs and the duo bond. Harold breaks a piece of his crayon and hands it to Mel.

The following day, Terri informs Harold and Moose that they must leave while Mel goes to school, so they do. However, Mel wants in on the mission to find Harold's dad and the trio of friends head to the local library and meet with the librarian, Gary Natwick, who struggles to pitch his long winding idea for a story and also admits to having a secret crush on Terri. They ask Gary if he can help find Harold's father through the computer, so he tries to no avail. When they leave the library, Harold draws an airplane and uses contrails, displaying Terris contact number in the sky and asks old man to call the number. Suddenly, Gary realizes what book Harold had come from. Terri has had loads of calls from complete strangers, followed by a call from the school, saying Mel had been involved in a bullying incident and to return home immediately. Harold and Moose spot Terri at work and Moose helps fill in for her by moving boxes, but get bored before long and begin to wreak havoc, ultimately getting Terri fired from her job. However, it is revealed that Terri didn't really like the job and that Moose had spared her a favor.

Harold, Moose, Mel and Terri walk into the town and Harold had found out in the garage that Terri wants to be a pianist, so he draws a piano for her to play. Gary meets up with Harold and Moose and explains that the "old man" he was referring to was actually the author of the book he was from. It is revealed that the old man's name was Crockett Johnson. Harold and Moose proceed to reconcile with Porcupine who had been chased down by the police for stealing a leather jacket. They are all arrested, but Harold draws a wrecking ball and the three escape. Porcupine uses the crayon to draw a car and drive to Crocketts house, only to find out that he has passed away. Harold is deeply saddened by this and all of his previous drawings begin to fade away, even Moose and Porcupine.

Harold meets Gary one to one and Gary convinces Harold to give up his crayon, to which he agrees. Gary uses the crayon to imprison Harold and cosplay himself as a fantasy warrior. Mel arrives soon after, only to become imprisoned as well. However, he uses his crayon part from earlier to free himself and Harold. The pair clash with Gary in the town square, where he is using the crayon to create his own fantasy world. Harold fights Gary for possession of the crayon, but Gary swallows the piece, before becoming super powerful, transitioning the grass into molten lava.

Mel uses his imaginary friend, who he drew at school earlier, named Carl, in an attempt to make Gary spit the crayon piece out, to which they succeed, turning everything back to the way it should be. Harold helps Gary out with his dream of living in his own fantasy world by drawing a door for him and he enters, before the door gets burnt by Carl.

Harold, Moose, Porcupine, Mel and Terri all visit a museum all about the work of Crockett Johnson and Harold reads a note and discovers the true meaning behind his creation. Harold, Moose and Porcupine return to their own dimension, but not before Harold passes Mel a pack of his very own Crayola Crayons, to use his own imagination with.