The sheer existence of this live-action adaptation of the classic children’s book “Harold and the Purple Crayon” inspires a number of questions on a basic level. Who is this for? The book by Crockett Jensen, published in 1955, is aimed at ages 3 and up, but the film, adapted by David Guion and Michael Handelman and starring an almost entirely adult cast, skews older. Yet the premise still feels too thin and juvenile to grab audiences of any age. So what algorithm decided this movie would be a lucrative endeavor?

I’ll take out my own purple crayon and attempt to magic a coherent film review into existence out of this profoundly odd and offputting film. The approach to adapting the beloved book is a somewhat curious one. We know the story and the iconography: a toddler in a onesie who uses his imagination and a purple crayon to make his world more exciting. In the film, our protagonist is now an adult, Harold (Zachary Levi), who still lives in his 2D world with his friends that he drew, Moose (Lil Rel Howery) and Porcupine (Tanya Reynolds). Despite being in his mid-40s, he’s still quite childlike and overly attached to his only parental figure, the narrator (voiced by Alfred Molina), whom he calls the “Old Man.” When one day the Old Man’s voice goes away, Harold draws himself a door to the real world in order to find him.

While the decision to feature a middle-aged Harold is above my pay grade, it likely has to do with Levi’s willingness — or desire — to star as a childlike man, as he has done previously in two “Shazam” films. His version of Harold is a cross between Tom Hanks in “Big” and another children’s book icon, Amelia Bedelia. Harold knows nothing of the real world, including irony, idioms or brands, and like Amelia, he takes everything literally, which is a problem when he’s wielding a crayon that can draw anything into being.

Perhaps it was cute for a 32-year old Tom Hanks to play a 12-year old boy, or even for Levi to slip into teenage-boy mode in “Shazam” (the first one, at least). But there’s something deeply strange about his performance here, grinning and mugging with childlike wonder in a way that can only be read as disingenuous.

For some reason, when they step into the real world, Moose and Porcupine become human, but I wouldn’t want to start asking questions to which there can’t be any good answers, so we’ll leave it at that. The trio cause light havoc, glomming onto a single mom, Terry (Zooey Deschanel), and her son, Mel (Benjamin Bottani). Harold, Moose and Porcupine need help finding the Old Man and then they get involved with Terry’s issues too (dead husband, dead-end job, wildly imaginative kid, lecherous librarian).

Things are looking dire in the world of “Harold and the Purple Crayon” (for me, specifically), and then a savior arises out of nowhere. It is the east and Jemaine Clement is the sun. He is here to save (or salvage what he can of) this film. Clement plays Library Gary, a writer of romantasy fiction, who has a crush on Terry.

Even though his pompous author character is a bit close to his role in the 2009 comedy “Gentleman Broncos,” Clement proves to be a breath of fresh air and a decent adversary. If Harold uses his crayon to pull planes and pies out of thin air, Gary has far more nefarious (and horny) intentions, using the crayon to summon up his fantasy world, complete with powerful magical staffs, a floor made of lava and scantily clad maidens.

It’s only when Clement brings his flair that this film has anything worth paying attention to, because he’s the dash of acid that offsets this otherwise totally saccharine and sappy story about embracing imagination. Gary adds a much-needed dose of friction to the whole endeavor, and Clement seems to be having fun, which is more than can be said for Deschanel, who brings a sort of dead-eyed “running lines” energy to her barely-there performance. The less we focus on what Levi, Howery and Reynolds are doing, the better.

The only actor committed to the heart of “Harold and the Purple Crayon” is young Bottani as Mel, because children more sincerely buy into the transformative power of imagination. As for why they decided to make this movie about — and for? — adults, you’ll need to tap into your own imaginative powers, if they’re not too busy elsewhere.

Katie Walsh is a Tribune News Service film critic.