“An adventure is only an inconvenience rightly considered. An inconvenience is only an adventure wrongly considered.” Prone to tantrums over the pettiest of setbacks, I’ve never been able to absorb this maxim of G. K. Chesterton, but I love it. And I love Pauline “Poppy” Cross, the heroine of Mike Leigh’s Happy-Go-Lucky, because she is the embodiment of the maxim.

Practically everyone and everything Poppy encounters arouses her curiosity, and her curiosity is tantamount to love. A thirty-something primary-school teacher in London, she seems incapable of dismissing anyone—child or adult—no matter how troublesome. The speck of wormwood swimming in the bloodstream of this bracing film is the realization (Leigh’s, and finally Poppy’s) that to embrace life without reservation is to invite the ill will of those who cannot. And those who cannot are legion. Most of the time Poppy may indeed be happy-go-lucky; the movie is not.

A word of warning: The first ten minutes may strike you as spinelessly naturalistic, promising nothing but an undramatized slice of life. Even I, who have admired other Mike Leigh movies, started to groan inwardly while watching the seemingly inane Poppy (with her off-putting chirp of a laugh and her terrible clothes—only Austin Powers could admire her stockings) first annoy a stolid bookstore clerk with her chatter (“Having a bad day?”), then get high with her girlfriends after club-hopping. Her apparently unbreachable cheeriness (when her bike is stolen, she can only wail, “I didn’t get a chance to say goodbye to it!”) made me fear I was in for an overdose of “Isn’t life wonderful even when it’s awful?” whimsy.

I couldn’t have been more wrong. When the bikeless Poppy decides to get a car and must learn how to drive it, Scott the Driving Instructor from Hell enters the picture and the movie becomes vividly funny yet fairly unsettling, with Poppy proving to be a true heroine—courageous, compassionate, and complex.

Frazzled driving instructors in charge of hopeless students have been a staple of comedy since comedian Bob Newhart’s first album, and I thought that Clueless (1995) had wrung the last possible laughs from the setup. But Leigh changes the chemistry of the teacher-student relationship. Yes, Scott is serious about the rules of the road, and yes, Poppy is flippant, but Scott’s humorlessness soon turns into bullying, and the bullying becomes a manifestation of a simmering madness—the madness of seeking control over others because you can’t control your own life. Scott’s teaching methods are really the protocols of anal-retentiveness (the triangle formed by the side- and rear-view mirrors is a mystical zone for him), and they are hilarious, until we realize that these rituals form a crumbling dike against the sea of his madness. Eddie Marsan’s performance is amazing. It never settles for mere vein-swelling fury but conducts us through layers of comedy, pathos, and terror.

Opposing Scott, Poppy’s flippancy—her wisecracks, flirtatiousness, and even the boots she wears in defiance of his demand for sensible shoes—comes across as justified rebellion, her life force defying his rigidity. Yet why should such defiance be necessary? Why doesn’t Poppy simply hire another teacher and get away from this pygmy fascist?

Before I try to answer that, consider two other scenes. Sometime after we have learned how conscientious a teacher Poppy is, we see her working near a school window through which she spots a child pounding a smaller kid in the schoolyard during recess. For a moment, she freezes and does nothing but stare. Later, she tries to find out what is behind the bullying and calls in social services to deal with an abusive family situation (an act that fortuitously brings romantic love into her life when the social worker proves to be attractive). But why did she freeze?

While walking home one evening, Poppy encounters a homeless man in one of those vacant lots that pockmark great cities. Whether because of drugs or madness, his mind is in a place far from Planet Earth. Drawn to him by his strange chanting, Poppy fruitlessly questions him, vainly tries to give him money, and then simply returns his stare. No passersby are in sight and, if the man should prove violent, she would certainly be in danger. Yet, when he briefly withdraws to the shadows to urinate, she waits patiently for his return so that their strange, one-sided interview can continue. Why?

I don’t have firm answers to these questions, but here’s my guess. Poppy, branded by her family and even some of her friends as a slaphappy optimist, is actually aware that something is loose in the world, something amorphous but clearly antipathetic to social comity and happiness; and that her goodwill and openness can only coexist with this something, not conquer it. This force dwells in Scott, in the disturbed child, in the homeless man. But just as she was briefly spellbound by the vagrant’s desolation and the violence of the child, Poppy is gripped by Scott’s furious otherness. In all these instances, it isn’t fear that brings Poppy to a standstill but a sort of muted awe, a breathless taking stock of an invading army that can only be defied, never conquered.

Sally Hawkins’s performance as Poppy has won international awards and will surely get an Oscar nomination. (If justice prevails, Marsan should get a supporting-actor nod.) Her objective, I think, was to establish Poppy’s nature so completely within her own imagination and nervous system that she could play the part without showy virtuosity. This is a performance not of outbursts but of glints, sidelong glances, nervous laughs, sudden forays, and sudden retreats. The actress simply brings Poppy into our lives and, if you are at all responsive to the movie, its heroine will stay with you until the day you die.

Mike Leigh’s camera behaves the way Poppy does: it constantly takes notice. At one point, Poppy and her new lover are gazing out over the city from an apartment balcony, with the camera positioned behind them for a medium shot. Nothing happens for a few seconds. Then Poppy slopes her body until her hip bumps the young man’s. Lovely. In another scene, our heroine and some pals are walking on a wharf in a seaside town. They pass a couple of massive, bearded, middle-aged blokes seated together, and for a moment, the two are merely the sort of extras who are positioned to fill the background in a movie. But Leigh allows Poppy and company to pass out of the frame and holds the camera on the two fellows. With their mountainous builds and their baggy garments, they begin to look interesting enough to have an entire movie to themselves. A beat later, they disappear from the film, and our lives, forever.

Given its title and the nature of its heroine, Happy-Go-Lucky was bound to be described as heartwarming and life-embracing. From time to time, it answers to these adjectives. But it is a good deal more: a resonant work of art.