Bel Powley in Marielle Heller’s “The Diary of a Teenage Girl.” Courtesy of Sony Pictures Classics.

There are about a million or so ways Marielle Heller’s “The Diary of a Teenage Girl” could’ve flown clear off the rails. While leaving the screening room, I ran into Michael Phillips of the Chicago Tribune, who told me that he had been waiting for the moment that the film would “go wrong,” and it never did. I likewise had found myself holding my breath as the filmmaker took one audacious gamble after another, tackling scenes of such profound discomfort that if the tone had been just a little bit off, it would have collapsed. By the end, I was in a state of awe. In her very first effort as a writer/director, Heller had triumphed in achieving what countless veteran filmmakers have attempted with underwhelming results. She portrayed the sexual awakening of a young woman without resorting to sensationalism, gross-out humor or sentimentality. This is one of the most authentic and insightful explorations of sexuality ever made. It is also the most liberating American film in recent memory, jettisoning us out of the Puritanical dark ages into a new era of enlightenment. Watching it is akin to observing modern American cinema lose its virginity.

Whereas “Twilight” and “Fifty Shades of Grey” feature female protagonists willing to deny their own identity while complying to the self-destructive lifestyles of their loathsome lovers, the titular hero in Heller’s film is not submissive in the least. 15-year-old Minnie (played by 21-year-old Bel Powley) is on an exhilarating journey of growth and discovery throughout the entire picture, and ultimately reaches a greater understanding of her own self-worth. The fact that she loses her virginity to her mother’s boyfriend, Monroe (Alexander Skarsgård)—a man 20 years her senior, may cause viewers to expect either a shattering drama reminiscent of David Schwimmer’s “Trust” or a squirm-inducing wallow in the darkness of human nature, a la Todd Solondz’s “Happiness.” Yet “Diary of a Teenage Girl” couldn’t be further from either of those examples. It regards Minnie’s deflowering from a matter-of-fact perspective, allowing the sincerity of her emotions to guide every scene. Monroe is clearly a sad sack, but Skarsgård plays the role without a hint of condescension. He seems to have genuine feelings for Minnie and has manipulated himself into believing his own delusional projection of her, until she ruptures it with the truth.

Powley’s performance is, quite simply, one for the ages. She may fleetingly resemble Patty Duke from some angles and Kate Winslet from others, but there has honestly been no one like her on film. To say that she is fearless would be an incalculable understatement. The film’s nudity is considerable though never exploitative, and Powley appears entirely comfortable in her own skin as she studies her reflection in the mirror, prances teasingly around Monroe in the bedroom and informs a bewildered boy her age, Ricky (Austin Lyon), that she prefers to be on top. Ricky fully expects her to be satisfied with his amateurish quick thrusts and is gobsmacked when Minnie takes control—switching positions and taking it slower, much to her increased pleasure. Yet it is Powley’s emotional nakedness, moment-to-moment, that is most remarkable of all. Somehow her vulnerability makes her exude an even fiercer strength as she embraces her newfound sexual power. The taped confessions she privately records and the explicit illustrations she draws are healthy forms of expression, enabling her to acknowledge and articulate every detail of her experience and thus be able to reflect on them with added clarity.

Heller’s Oscar-worthy script is adapted from Phoebe Gloeckner’s 2002 book of the same name, which garnered a mixture of acclaim and controversy that will likely mirror the mainstream reception for the film. One of Gloeckner’s masterstrokes was the way in which the book resembled an actual diary, combining intimate prose with vivid imagery resembling that of a graphic novel. Rather than render this approach as a cute gimmick, Heller has meticulously created the cinematic equivalent of a teenager’s diary, complete with animated, hand-drawn flourishes that beautifully accentuate the hormonal storm reverberating within Minnie. The film suggests what “American Splendor” may have been like, had it not focused on the mind of a misanthrope. Minnie’s exuberance is infectious from the very first frame, as she strides through a park, confessing via voice-over that she “had sex today…Holy S—t!”, as the film’s marvelous, period-specific soundtrack pulsates through the sun-drenched landscape. The film’s setting is 1976, a crucial factor in the characters’ evolving views regarding feminism and relationships, yet the story is ultimately a timeless one, resonating especially strongly at a time when sexual shaming and repression are still distressingly commonplace throughout the country and around the world.

Sex is not the enemy in “Diary of a Teenage Girl,” it is merely the vessel with which Minnie comes of age. If she had suppressed her sexuality in the aftermath of her time with Monroe, her growth would have become irreparably stunted by undeserved shame. Minnie’s decision to continue delving further into her sexuality, experimenting with her identity while making mistakes (and learning from them) along the way, has made her victorious. Her final, delicately nuanced encounter with Monroe is one of the most well-written and acted moments I’ve ever seen. It finds the right note and hits it out of the park with a magnificent punchline that caused an entire screening room of critics to let out an audible burst of elation. In many ways, Minnie embodies the honesty, wit and courage of the trailblazing women currently defying the conventions of a shamefully sexist industry: Amy Schumer, Lena Dunham and Joanna Arnow, to name a few. These women refuse to conform to Hollywood’s stereotypical definition of the female form, and they present themselves as unapologetically sexual beings worthy of love, while tackling issues of gender and identity that many of their male counterparts have timidly neglected. Minnie’s unabashed resilience encapsulates their spirit.

There’s one more performance here that I must single out. Minnie’s mother, Charlotte, is played by Kristen Wiig, a woman whose comic genius first struck me when I witnessed the brilliantly loopy stream-of-consciousness monologues spewing from her mouth as Judy Grimes, the deeply neurotic travel correspondent on “SNL.” She could’ve made an easy fortune with the money provided by a needless “Bridesmaids” sequel, but instead has taken on one challenging role after another in indies such as “The Skeleton Twins,” “Hateship Loveship” and “Welcome to Me.” The role of Charlotte is an especially tricky one, considering the enormous blind spot she must have in order to shield Monroe’s multiple red flags from view. Her ex-husband, Pascal (a hilariously aloof Christopher Meloni), accurately sizes up his wife’s lover, but is too inept a father to be of much help. Charlotte’s own sexual openness does not make the revelation of her boyfriend’s indefensible actions any less difficult to stomach, and Wiig deftly navigates through her character’s cocktail of outrage, betrayal and guilt, while never failing to locate the undying love she has for her daughter.

“Do I look different than I did yesterday?” Minnie asks her cat in the hours after bidding her virginity adieu. The answer would seem to be, “No,” at first glance, considering she still has the typical mannerisms and affect of a 15-year-old girl. Nothing has changed and yet everything has, all at once. That is the nature of adolescence—and of life.

“The Diary of a Teenage Girl” opens in Chicago on Friday, August 14th.