Jessica Yu’s “In the Realms of the Unreal” is in style and content different from any other documentary I’ve ever seen. Ostensibly a biography of Henry Darger, a reclusive savant, “Realms” is more an exploration of the strange gray area where an author’s life and his work intersect. Yu leaves just enough unsaid in her exploration of Darger’s inner life, resulting in a film that is thoughtful and intriguing. Best of all, she treats her material with subtlety, whereas a director more determined to come to decisive conclusions might have spoiled some of the mystery with heavy-handed judgement.

Henry Darger was a janitor who lived in Chicago during the first half of the twentieth century. He was a recluse his entire life; he had one friend and few acquaintances and rarely left the few blocks between the Catholic hospital where he worked and the one room apartment in which he lived. When Darger died, his landlord found an enormous, 15,000-page fictional epic that Darger had apparently spent his entire life writing. Entitled “The Story of the Vivian Girls, in What is Known as the Realms of the Unreal, of the Glandeco-Angelinnian War Storm, as Caused by the Child Slave Rebellion,” the work painstakingly records every detail of a long civil war that rages between the Christian forces of good and their dark enemies in a fantasy world.

Accompanying the book were gigantic otherworldly murals drawn on butcher paper, scenes of naked little girls with male genitalia, and evil soldiers in Confederate coats and graduation caps, all vividly colored and incredibly detailed. Darger’s immersion in this universe was nearly complete. He rarely slept, preferring to spend nights keeping detailed records of casualties and combatants as the fictional war ground on ceaselessly during the long years of his life.

The brilliance of Yu’s documentary is that she largely relies on Darger’s own words and paintings to tell his story. Dakota Fanning (“I Am Sam,” “Hide and Seek”) reads excerpts from Darger’s journals and his novel, while the camera pans past his images — which Yu brings to life with wonderful animation. This technique exposes the parallels between Darger’s life and fiction: his mistrust of adults, the time he spent at the Illinois “State Farm for Enfeebled Children,” his brief stint in the army, and his lifelong struggle with Christianity. Darger’s story is not told through government records or the testimony of outsiders but revealed as he wrote about it in surprisingly lucid journals and hallucinogenic fiction.

Yu’s presence in the film is barely felt. Using a technique that is the antithesis of Michael Moore’s, she never asserts an opinion about Darger’s controversial art, nor does she even settle on a likely explanation for what motivated him. (We do not even hear her voice once.) Though Darger has been called everything from a brilliant artist to a pedophile, Yu refrains labeling the man at all, rather treating him as the fascinating, puzzling man that he was.

The only time she leaves Darger’s mind is in interviews with his neighbors and landlord. But the first time we hear their voices is when Yu plays a clip of each person pronouncing Darger’s last name differently, an effective warning that no one truly knew him. As a result, the film is less a straight-ahead biography of the artist and more a hypnotic journey into his mind.

There is very little wrong with the film, although Fanning’s voice is so pinafored and pigtailed as to be sickeningly cutesy at times. In the same vein, Yu occasionally succumbs to the banality of clips of vintage Chicago tourism ads that look good, though don’t really add much to the study of Darger’s psyche. But the narrator’s exaggeratedly childish tone, and the banners praising Chicago as the City of the Future fade quickly as we are engulfed in the artist’s eerily beautiful world.

Many might think that Yu’s lack of explanation for Darger’s actions is often frustrating. Why did he feel the need to record the weather and compare it to the newspapers’ weather reports for nearly ten years? Why did he write two endings to his epic myth, one in which the forces of evil prevail? Why did a man who clearly worshipped young children frequently illustrate them being eviscerated in his murals? Ultimately, the film’s lack of explanation — which goes hand in hand with its lack of judgement and moralization — is its greatest asset. It is at times maddening, but Yu knows what she is doing: you’ll ponder the mystery of Henry Darger’s life long after you leave the theater.