As the 30-something Maggie (Greta Gerwig) bathes opposite her towheaded 2-year-old daughter Lily (Ida Rohatyn) in a bubble-bath filled with floating toys, she makes an observation: “I want to live inside a bubble.” But as we’ve discovered in the scenes leading up to this one, the protagonist of Maggie’s Plan has already been living in a bubble—and it’s about to burst.

The fifth film from writer/director Rebecca Miller, daughter of playwright Arthur Miller, finds her building on an unpublished novel by her book editor friend Karen Rinaldi, the Other Woman in Catherine Texier’s 1999 memoir, Breakup: The End of a Love Story. In Miller’s refashioned dramedy, Maggie’s plan is actually two separate ones, spawned from what her best friend Tony (a shaggy-haired Bill Hader) terms her ill-fated “best intentions.”

When Miller introduces us to Maggie, it’s a few years earlier, just before the best-intentioned meddler meets cute with John Harding (Ethan Hawke) when she’s issued one of his paychecks by mistake. They both work at the New School in New York—Maggie as an administrator, John as a “ficto-critical anthropologist”—and the mixup occurs because of the similarity of their names. Maggie is actually one Joanna Margaret Hardin, and her first plan involves having a baby through artificial insemination, since our Quaker-raised heroine feels she will never find love or be mature enough for marriage. She might be right about that second part.

Maggie has lined up a donor possessed of a self-described “genetic goldmine”—a guy named, well, Guy (Travis Fimmel of TV’s Vikings). He’s an artisanal pickle maker (an actual profession, unlike, say “ficto-critical anthropology”), and before this genteel millennial hippie can answer when she asks how much involvement he might like to have in the life of her child, she provides her own: “I would suggest none.”

The plan proceeds, as does Maggie’s newfound infatuation with John, who dreams of supplanting his stagnant career as an adjunct professor in a made-up field with an attempt at becoming a novelist. But as the saying goes, the best-laid plans often go awry. When John asks Maggie to take a look at his work-in-progress, she not only loves his writing, but falls for him as well. So what if he’s married with two children?

Jumping ahead a few years, Maggie has become a minivan-driving mom to three kids—including her older stepchildren, Paul (Jackson Frazer) and Justine (Mina Sundwall)—or four, if you include the middle-aged John, who avoids adult responsibilities by focusing his time and energy on the now-massive novel he may never finish. Not only that, but John’s Danish ex-wife Georgette (Julianne Moore), a tenured professor at Columbia, is about to publish a memoir dealing with the dissolution of her marriage, with Maggie as a featured villain.

Nevertheless, Maggie tries to stay the course, extending an olive branch to Georgette, who gains an appreciation for the woman who broke up her marriage. “It’s funny,” Georgette says in Moore’s comically clipped cadence. “You’re a good person…a little bit stupid.”

But then, it would take a touch of stupidity to come up with the second plan that Maggie hatches. Always wanting to do the right thing (to a fault), she orchestrates a scenario that she hopes will end with John returning to Georgette, restoring order to lives she feels she’s disrupted. It’s here that the film slips into screwball gear, and the picture is much better for Moore’s larger presence, since her heavily accented caricature of the woman scorned serves as a great foil for Gerwig. Can Maggie rebuild the home that she’s wrecked, restoring balance to lives that may have worked better before she entered? That’s the hook that Miller’s film hangs on, but it’s no guarantee for big laughs, even with the presence of Wallace Shawn, along with Saturday Night Live alums Hader and Maya Rudolph (as Tony’s wife, Felicia), who serve as the picture’s Greek chorus.

Miller is known for dramas such as 2002’s Personal Velocity, 2005’s The Ballad of Jack and Rose (which starred her husband of two decades, Daniel Day-Lewis) and 2009’s The Private Lives of Pippa Lee, her semi-autobiographical attempt at using fiction to free her photographer mother from Arthur Miller’s more famous shadow. As such, screwball comedy and snappy dialogue aren’t exactly her forte; one can’t help but wonder what Gerwig and Frances Ha co-writer and director Noah Baumbach—her creative and romantic partner—might have accomplished with Maggie’s Plan’s premise. But Miller’s obvious intelligence and humane insights nevertheless provide us with a character who manages to free herself.

Unlacing her role as “Little Miss Quaker Two-Shoes,” Maggie eventually loosens up enough to embrace the unpredictable, inevitable role of destiny, not only on her own life, but on the lives of those around her. Miller accomplishes this with good humor, if not guffaws, aided immeasurably by a generous ensemble of actors who project decency, even when their characters make questionable decisions.

Maggie’s Plan ***1/2

Greta Gerwig, Ethan Hawke, Julianne Moore, Maya Rudolph, Bill Hader, Travis Fimmel, Wallace Shawn, Mina Sundwall, Jackson Frazer, Monte Greene and Ida Rohatyn. Written and directed by Rebecca Miller, based on a story by Miller and Karen Rinaldi. At Boston Common, Coolidge Corner, Kendall Square and in the suburbs.