Sally Hawkins (2008's "Cassandra's Dream") is a force of nature in "Happy-Go-Lucky," one of the lightest and frothiest efforts from British writer-director Mike Leigh in years. Leigh often likes exploring dark emotional subject matter, but here, he has chosen a lead character who is so friendly, bubbly and upbeat that it would probably take her own death to put a damper on her cheery mood. Hawkins plays this role of Poppy with such an affable go-getter quality that the actress molds herself completely into the character she is playing. You, the viewer, instantly love her for her unique goodness. Unfortunately, this is the only thing the film has going for it.

There is no plot to speak of, just a bunch of scene sketches. Waiting for a central conflict or story through-line to kick in is fruitless. Poppy (Sally Hawkins) is a 30-year-old elementary school teacher, happily living the single life with flatmate Zoe (Alexis Zegerman). The film follows her in her everyday routine. When her bicycle is stolen ("I didn't even get to say good-bye!" is her audible disappointed reaction to the theft), Poppy decides it is high time to get her driver's license. In between her weekly lessons with edgy instructor Scott (Eddie Marsan), who secretly pines for her, she attends flamenco classes, occasionally gabs and clubs with Zoe and their friends, investigates a troubled student who is being abusive toward his classmates, and tentatively begins a romance with social worker Tim (Samuel Roukin).

The biggest obstacle Poppy faces in "Happy-Go-Lucky" is the realization that not everyone she comes into contact with is as beguiled by her as she assumes. While questioning her level of happiness—there are tough times in life, she decides, but yes, she is—she is forced to examine the person she is. Is she too forceful and overbearing? Too juvenile still? That Poppy considers these things and decides that she likes herself just the way she is, thank you very much, is the picture's most winning element.

Well, that and Sally Hawkins' resplendent, tartly funny performance as Poppy. With more conventional writing, Poppy would be a caricature—an increasingly wearisome bubbly ball of giddiness—and nothing more. That she allows herself time for introspection, as when she has a run-in with a homeless man, or when Scott harshly critiques who he sees her as, she grows to be more layered and ultimately admirable than the chatty, smiling face she comes off as in the first scene. She's not perfect and she doesn't believe she is, even when it might come off that way to outsiders looking in.

If Hawkins is terrific, writer-director Mike Leigh gives Poppy little to do that is engaging. The supporting characters do the limited amount required of them, and side stories, like the pupil Poppy fears may be having troubles at home, don't amount to much. Poppy's relationship with Tim is easygoing and they seem to have nice chemistry, but such a little deal is made about it that its impact is fleeting. Meanwhile, the film turns serious when Scott reveals personal problems that go far beyond his unrequited crush on his colorfully-dressed wise-cracking driving student, but this, like the rest of the proceedings, are treated fluffily. And that, alas, is the problem. "Happy-Go-Lucky" is cute but inconsequential, pleasant to a degree but so willowy that it hardly seems to exist at all. There may very well be a great motion picture to be made that incorporates the one-of-a-kind Poppy, but this one isn't worth her (or our) energy.