Away we go, Edinburgh International Film Festival, Edinburgh

A safe trip down a well-worn path

From American Beauty to this year's Revolutionary Road, Sam Mendes has made it his business to explore the trials and tribulations of domesticity. As the curtain-raiser for this year's Edinburgh International Film Festival, his new movie, Away We Go, continues this obsession, albeit with Mendes shifting down a gear to deliver a comic travelogue that finally gets him out of the suburbs. Forget Two Lane Blacktop and the like; what follows is probably the most right-on road movie ever made.

Penned by husband-and-wife authors Dave Eggers and Vendela Vida, the plot follows unmarried thirty-somethings Burt (John Krasinski) and Verona (Maya Rudolph) as they come to terms not only with impending parenthood, but with the need to find their place in the world. With Verona six months pregnant, the Colorado couple are preparing for the life-changing experience when Burt's parents (Jeff Daniels, Catherine O'Hara) suddenly decide to take the chance to relocate (somewhat bizarrely) to Antwerp.

Shaken by this, not least because they'd moved to Colorado to be near Burt's parents (Verona's died in an accident years earlier), the couple decide to take off on a road trip to find a more suitable place to nest. Their first stop is Phoenix, where Verona's former colleague Lily (Allison Janney) now lives with her husband and two children. Vulgar and crass, Lily is a lush who lets inappropriate comments spill out of her mouth with alarming regularity (at one point, referring to her young daughter, she says, "this one has that dyke look – she looks like a teamster").

Undeterred, their next stop is in Tucson, where they briefly hook up with Verona's well-adjusted sister, Grace (Carmen Ejogo), but no sooner have we just got to grips with her, than its on to Madison, where Burt's childhood friend Ellen – or LN as she prefers – now resides. Played by Maggie Gyllenhaal, this University of Wisconsin professor of women's studies is like the Earth Mother from hell. When it comes to child rearing, when she isn't breast feeding other people's children, she abides by the "3S" rule: "no separation, no sugar, no strollers".

Like any road movie, particularly one that skips across rather humdrum terrain as this does, you're only as good as those you meet. And in the case of Away We Go, the characters that populate this particular journey veer dangerously towards condescending caricature. While the likes of Janney and Gyllenhaal go full-throttle, drawing plenty of laughs in their brief scenes, they're never given enough screen time to establish themselves as anything other than comic contrivances. The episodic nature of the script hardly helps either, never allowing a natural rhythm to be established as we hop from one state to the next.

It's not hard to see what Mendes is trying for here. As they blithely skip around North America – Montreal and Miami are also on the itinerary – the all-too-perfect Burt and Verona (despite early protestations that they may be "fuck-ups") are made to look entirely normal by the grotesque examples of damaged relationships that they encounter. By the time they meet Burt's brother (Paul Schneider), who has just been abandoned by his wife, in what is arguably the most touching sequence in the film, it's a relief to encounter someone relatively normal.

Scored with a selection of cuts from the Scottish singer-songwriter Alexi Murdoch, thankfully the film is anchored by two solid performances from its leads. The likeable Krasinski (Jim, the "Tim" figure from the US version of The Office) and the low-key Rudolph make for a convincing pairing and it's their commitment to the cause that also ensures the final scene is quietly moving. But while there are lines to savour, by Mendes' own high standards, Away We Go feels like a meandering doodle. Lacking his usual sharpness, he treads not so much a revolutionary road here, as an overly familiar one.

'Away We Go' is released on 18 September