There is a spark of cosmic whimsy and strong, but not uninvited, hints of Woody Allen percolating in the DNA of "Maggie's Plan." Written and directed by Rebecca Miller (whose literary anthology "Personal Velocity" won the U.S. Dramatic Competition's Grand Jury Prize at the 2002 Sundance Film Festival), this ample-humored romantic comedy requires a certain suspension of disbelief when it comes to its generously placed serendipitous narrative turns, the kind that must happen in order to move the story forward. With the wrong touch, these contrivances could destroy one's good will, but viewers are in safe hands with Miller at the helm. Her latest endeavor is on the fluffier side, but intelligent in the way she writes her three vibrantly imagined protagonists from an unbiased angle. While they are all guilty on some level for mistakes and indiscretions of the past, they are also good people who want to do the right thing—even if they tend to go the wrong way about it.

The always-radiant Greta Gerwig (2013's "Frances Ha") stars as Maggie Hardin, a gung-ho student advisor at NYC's The New School who unexpectedly falls in love with unhappily married ficto-critical anthropology professor John Harden (Ethan Hawke) just as she has begun the artificial insemination process. Three years later, Maggie and John have wed and had a daughter, Lily (Ida Rohatyn), but their relationship has cooled considerably. When Maggie meets his ex-wife, fiercely ambitious academic Georgette (Julianne Moore), she is surprised to discover she kind of likes her and doesn't seem to be the nightmare John semi-autobiographically had described in his long-in-the-works novel. Suspecting that John and Georgette still love one another and feeling guiltily like a home-wrecker who tore apart their family unit—they now share custody of their kids, Justine (Mina Sundwall) and Paul (Jackson Frazer)—Maggie concocts a scheme to get the divorcees back together and make things right.

"Maggie's Plan" is breezy if a little too arch at times; the opening-scene setup where Maggie tells best friend Tony (Bill Hader) about her plans to artificially inseminate with a former classmate, pickle entrepreneur Guy (Travis Fimmel), awkwardly dives into exposition that would be more appropriate for the stage. The introduction of Georgette—a true original with a curiously affected Danish accent—also takes some getting used to, but within a couple scenes Julianne Moore (2014's "Still Alice") has come into her own, threatening to steal the film when she's onscreen. Moore's droll comic delivery and haughty-but-human demeanor earns a number of big laughs, particularly when she and John find themselves stranded on a business trip at a snowy Quebec resort where Maggie has orchestrated their reunion. As John, a rakish guy who insists he still loves Maggie even as he inadvertently neglects her needs, Ethan Hawke (2014's "Boyhood") avoids turning his character into a lout and remains an affable, sincere presence. As self-deprecating married friends Tony and Felicia, Bill Hader (2014's "The Skeleton Twins") and Maya Rudolph (2015's "Sisters") lend welcome sounding-board support.

Even as the plot occasionally meanders, it is ultimately Greta Gerwig who carries "Maggie's Plan" to its unlikely yet sweet denouement. There are no other actors who have Gerwig's very specific quirky spark, and few who are as instantly ingratiating. A less astute script with a less likable lead might have spelled disaster for the film's eponymous heroine; she does, let's face it, play a part in breaking up a marriage. Writer-director Rebecca Miller understands and listens to Maggie, and Gerwig plays her as a capable young woman with a gentle soul, a book-smart savviness, and a slightly scattered undercurrent. She means well, even when her master designs aren't exactly airtight. Seeing how "Maggie's Plan" unravels its tangled love triangle and plays for redemption is fun while it lasts.