Juno is the type of movie that lives or dies by its tone. I spent at least a third of it wondering whether its aggressively idiomatic dialogue would ring as strained to the teen cohort of its protagonist as it did to me. The other possibility is that those same kids will gravitate toward a character whose mouth is appreciably smarter than theirs, even if it's also often too clever -- or canned -- by half.

Directed by Jason Reitman, Juno is the story of Juno MacGuff (Ellen Page), a high school junior and card-carrying wiseass who lives in a town so small that the counter guy at the five and dime knows you're knocked up as soon as you do ("Your eggo is prego," says Rainn Wilson in his special brand of manic deadpan). Juno's eggo is prego, and the father is her friend Paulie Bleeker (Michael Cera), a sweet-eyed naïf she seems to have deflowered for Sunday afternoon kicks. Juno's father (J.K. Simmons) and stepmom (Allison Janney) typify the inadequate response that meets Juno's news at every turn, perhaps because it's understood that fuss will not be tolerated; Juno hides behind her flannel and flint, tough-little-nut persona, avoiding drama -- and perhaps actual emotion -- at all costs. When she decides to give her baby up to an affluent suburban family, her parents and the hapless father can only shrug and follow.

That decision follows a rather silly trip to a Planned Parenthood clinic ("I'm just calling to procure a hasty abortion"), in which Juno is offered boisonberry condoms by a gum-snapping receptionist and balks when she finds out that at some point a fetus develops fingernails. It feels like yet another inadequate response to Juno's pregnancy -- this one the script's -- though the film gets credit for at least using the word and envisioning such a trip, unlike Knocked Up, Waitress and Stephanie Daley. Lord knows I saw enough girls waddling around my (über-Catholic) high school to know it's not an unusual decision.

The clinic trip (the receptionist endorses the condoms chiefly for the pleasant scent they give her boyfriend's nethers) is a prime example of Juno's tonal overreaches, the preponderance of which occur early on. Juno's voiceover, sometimes genuinely funny (she speculates that the staunchest jocks secretly covet "girls who play the cello and read McSweeney's and want to be children's librarians when they grow up"), also strays into the self-consciously tart, heavy not just with tone but "tone"; it suffers, then, from the pitfalls of the heavily autobiographical first script. Indeed it is former stripper Diablo Cody's heavily autobiographical first script, in as much as Juno's voice is meant to be her own, and Page has got the flat, downward inflections of a self-identified smack-talking, deflectively tough girl nailed. It's the words that ring false; when Cody eases up on the "tone," ironically, the film finds a more genuine and intuitive tone.

Juno selects Vanessa (Jennifer Garner) and Mark (Jason Bateman) as parents for her baby, a local couple living in one of those suburban castles of white and beige. Vanessa, desperate for a baby, has too much to lose to be amused by Juno's pathological one-liners, wielding her dimples with luscious, tactful concern; Mark takes the time to be tickled by Juno, and the two bond over bons mots, mix CD's, affectionate insults about their mix CD's (he's a 1993 guy, Juno is stuck in 1977) and several of my favorite songs, making me wonder if I am supposed to be as old as Mark is. Juno waits out her pregnancy like it's a toenail infection, growing somewhat inappropriately close to the clearly lonely and terrified Mark, and failing to recognize her outrage at Bleeker taking a girl who "smells like soup" to the prom for what it is.

Reitman does a nice job dividing Cody's free-ranging script into a series of nuanced, tightly-framed conversational moments, rotating different configurations of the players through the tiny, two-shot moments that wind up changing lives. When a heavily pregnant Juno runs into Vanessa at the mall, Vanessa, who wants this so badly she can't quite see straight, is moved to tears by the baby's kick, and Juno's blithe façade is palpably pierced by the would-be mom's intensity.

It's an involvement that gets even more complicated when Juno gets caught in the ugly reality of a mature relationship -- Mark and Vanessa's -- who are themselves struggling with the difference between being prepared and being ready for such a life-changing experience. It's a nice surprise to watch Juno, who trafficks, if lightly, in the usual teen angst bullshit, get a taste of actual disillusionment, and Page does a solid job with some fairly tricky scenes.

Though the dynamic between Juno and Bleeker is not explored to much satisfaction, the tentative sweetness of Cody's ending feels earned; Moldy Peaches singer Kimya Dawson's quirk-alert score may not be for everyone, but the closing duet hits a perfect note: "I don't see what anyone can see in anyone else but you." Because nothing says "high school" like misanthropes in love.