It's never easy to synopsize a Mike Leigh film. For some 40 years now, the rightfully legendary British writer-director has honed a method of developing his characters and their stories through elaborate improvisations and exercises with actors, which he the transmutes into concisely scripted movies whose subtexts and backstories are so rich and multicolored as to feel almost miraculous. Particularly in the indelible works he's made in the past 20 years—including High Hopes, Life Is Sweet, Secrets & Lies, Topsy-Turvy, Vera Drake, and masterpiece-among-masterpieces Naked—there's so much detail beneath the surface that each subsequent viewing reveals new layers of possibility.

But to extol Leigh's subtext is not to disparage his text. It's just to say that his surfaces can be deceptively simple. The drama he deals in is internal, measured on the scale of human relationships—between parents and children, husbands and wives, workers and bosses, individuals and institutions. These matters can look insignificant set against the explosive multiplicity of entertainment options, which is why Leigh's radiantly varied body of work tends to be marginalized, even among art-film crowds; a Mike Leigh film is always a Mike Leigh film, for good or ill.

His latest achievement, Happy-Go-Lucky, will be no exception. It's another breathtaking Mike Leigh film, nearly impossible to condense. (Want evidence? Try the film's trailer, easily the worst preview for a great movie in recent memory.) To say that Happy-Go-Lucky is about an infectiously optimistic primary schoolteacher named Poppy who takes some driving lessons from a high-strung racist, drinks a lot with her beloved roommate, takes flamenco lessons from a fiery Latina, and finds a perfect boyfriend while negotiating the travails of contemporary London—well, it's not inaccurate. But such a breakdown doesn't begin to suggest the complex inner life of the film or of its heroine, who is indisputably a heroine and not just a leading lady.

Though Poppy's most obvious trait is that she is tirelessly, almost politically happy in the face of major and minor adversity, that happiness is a kind of armor, a vehicle for self- preservation—and the preservation of others—in an increasingly joyless world. After a couple of opening scenes that make you wonder if she might not be more annoying than charming, as willfully cheery people so often are in life, she becomes positively captivating. That's because her optimism isn't vacuous. It's always a choice. And even when it puts her in harm's way, it's always the right one.

Poppy is played—better to say inhabited—by Sally Hawkins, who gives the kind of performance that seems impossible under any circumstances other than a Leigh film, in which such body-and-soul idiosyncrasy, grounded by clear humanity, tends to be the norm (Eddie Marsan and Alexis Zegerman also give extraordinary, entirely distinctive performances). A jumble of clashing pastel colors and ludicrous plastic accessories, Poppy is impossible to ignore. Though her wardrobe may seem hyperintentionally zany at first—she dresses like a girl playing dress-up—it's clear (and not just from her bright pink bra and bright orange panties under black fishnets) that she is all woman. If her clothes were chosen to delight the kids she teaches, she wears them outside the classroom because they clearly delight her just as much. A bump in the road on the bus is enough to make her giggle, but when confronting a school-yard bully or her own attacker, her blithe voice drops a register and she becomes all business.

Since Poppy is the center of the movie's universe, its look and design radiate outward from her. Startling bursts of color leap from an outdoor market: a red dress hanging on a stall, a pair of bright yellow pants, Poppy's azure frock, her sister's rainbow jacket. These colors aren't merely decorative; they form a corona around our luminous heroine. The further away things are from her the colder they become.

Leigh's visual aesthetic has grown increasingly ambitious. Happy-Go-Lucky is his first film shot in widescreen, and despite the relative absence of panoramic vistas, the decision fits. Poppy exerts a gravitational pull on the vivid, dark, pancultural London in which this film is set. Her friends love her. Her driving instructor is infuriated and tempted by her. Her students are enthralled by her. She even reaches out to a vagrant so damaged by life he can't finish a sentence—all this in a widescreen urban context into which someone so tiny would likely disappear if she didn't take such pains to matter. And matter she does. Poppy's positivity might seem a naive approach to life in a world as blighted as this one. But having seen her in action, you can be forgiven for thinking that a person who chooses to enjoy her life, caring deeply about others, investing her considerable energy in being a loving teacher and a loyal friend, and adding color to a dying world, might understand something the miserable, angry rest of us don't. recommended