There’s something incredibly endearing about Infinitely Polar Bear’s unostentatious simplicity. Constructed on familiar indie tropes – a hand-made, whimsical aesthetic, a period setting, a family unit defined equally by conflict and closeness – the film sidesteps cliché to conjure an utterly charming experience.

Infinitely Polar Bear’s impossibly twee title is paraphrased from Faith Stuart’s (Ashley Aufderheide’s) description of her father, Cam (Mark Ruffalo), as “totally polar bear” – meaning bi-polar, or manic-depressive. Cam’s mental illness serves as the dramatic impetus for the film; after warm introductory narration from older daughter, Amelia (Imogene Wolodarsky, daughter of writer/director Maya Forbes), we’re deposited into the middle of his latest breakdown in the summer of 1978 – involving a jaunt in the woods, a bit of biffo with his boss and a call to the police when his wife, Maggie (Zoë Saldana), tries to flee with their children. But manic-depression is baked into the very bones of Infinitely Polar Bear, which swerves between small-scale drama and feel-good slice-of-life. It sounds precarious, but careful judgment ensures that the film balances these two poles throughout.

This isn’t the kind of film that makes big statements about society, or builds to a shocking climax that ties its themes together neatly. It’s a modest, observational film, content to portray the events of this family’s life in a way that suggests its politics without making them explicit. After Cam’s breakdown – and unemployment – the family moves to a rent-controlled apartment in Cambridge, and soon Cam becomes the primary guardian of his two daughters as Maggie completes her MBA interstate in New York. Forbes – who loosely based the story on her own childhood – dances through issues of mental illness, poverty, sexism and racism without allowing them to over-shadow the simple pleasure of living with these people.

Most family dramas – even great ones – struggle to provide an inclusive depiction of their subjects. A focus on children will reduce their parents to supporting characters, and vice versa. The small miracle of Infinitely Polar Bear is how it manages to give each of its character’s perspectives equal weight. The nature of Ruffalo’s character – gregarious, unpredictable, emotional – ensures that he remains front and centre, but the central conflict between Cam and Maggie is moving because we sympathise both with Cam’s absolute love for his wife and Maggie’s inability to endure his erratic behaviour. Amelia and Faith are not rarely-seen plot devices, but intelligent, assertive individuals who are capable of loving their father absolutely while protesting loudly at his often inexplicable behaviour.

This is thanks in large part to the exemplary work of the actors. Ruffalo is one of our generation’s most consistent actors, but his flagship roles thus far have tended towards subtler performances of gravitas, strength and reserve. Here he’s given the kind of showy role that attracts awards attention and underplays it masterfully, capturing his character’s off-kilter mania without ever reducing him to a stereotype. It’s perhaps the best work of his career. Saldana – in the rare starring role that doesn’t require her to paint her skin blue or green – is equally excellent, embodying a twisted physicality suggesting her internal conflict. Most impressive, in my book, is the work of Wolodarsky and Aufderheide, who each deliver convincing naturalistic performances demonstrative of talent far beyond their years. Keep an eye on these two.

Forbes’ direction, with the aid of cinematographer Bobby Bukowski, approaches the material with a suitably unshowy approach. The gentle, overexposed photography and unpredictable yet purposeful editing rhythm recalls the work of Hal Ashby. The whole film resonates with the texture of ‘70s cinema – appropriate to its era – but has an immediacy and agility, presumably thanks to modern digital photography, that harmonises with the everyday rhythms of the Stuart family. The net effect is charming and engaging in equal measure; it’s simply a pleasure to watch.