There is nothing truly surprising in The Black Balloon, only affecting riffs on a familiar story.Thomas (Rhys Wakefield) is a typical teenager, handsome, shy around girls, and innocent enough that a peck on the cheek packs the power of a defining moment. His father (Erik Thomson) is a military man, cheerfully gruff but no rigid taskmaster. His very pregnant mother (Toni Collette), though immobilized by her globe-shaped belly, remains the doting queen of the household. And his brother Charlie (Luke Ford) is autistic.

His is not the fit-for-popular-consumption autism of Rain Man. Charlie, we learn, could speak when he was young, but as a teenager roughly the same age as his brother, he can only grunt and shriek. His temper tantrums are fearsome and very often violent, as difficult to quell as they are to predict.

Inspired by her experiences growing up with two autistic brothers, director Elissa Down shows how Charlie’s autism, compounded by attention-deficit hyperactivity, takes its toll on his parents, who suffer his fits with exasperation tempered by genuine compassion, and Thomas, who feels increasingly alienated. Thomas dreams of a normal childhood for Charlie and himself, and is exhausted by the physical and emotional demands of being his brother’s keeper.

Adding to his frustration is Charlie’s potential to sabotage a burgeoning romance with Jackie (Gemma Ward of The Strangers), a sweet, pretty girl from school. Jackie meets Charlie under unusual circumstances -- he runs into her home and uses the bathroom, with Thomas in hot pursuit to stop him -- but she seems to understand the situation immediately, treating him with kindness and endless understanding. Thomas, who tries to keep his brother a secret, shows far less patience, though understandably so.

The Black Balloon is just as interested in Thomas and Jackie’s courtship, which is disarmingly unaffected, as it is with the sometimes uneasy dynamic between Thomas and Charlie. The two are intertwined, of course. Thomas wants what all boys his age want but feels saddled by a burden he can never shake. The movie seems to be asking a simple question – how would you handle it?

There is poignancy in Thomas’ struggle to find an answer. Charlie is an uncontrollable force of nature, and though he cannot express himself in words, his ups and downs are easy to read. Early on, Down shows him for the unwitting menace he can be, perhaps at too much length -- the point feels hammered home, though it helps us to understand his brother’s impatience later. Still, there is little doubt where their story is headed.

Thomas and Charlie’s is the ultimate love-hate relationship, and while they manage to work through their differences, they stumble often along the way. No great surprise there, but give The Black Balloon credit for taking a subject some filmmakers might consider untenable and treating it with the depth of feeling it deserves.