We first encounter him wandering the streets as if he was a toddler discovering the outside world for the first time. He soaks in the rain. Feels the ground beneath his feet. And then he finds himself more or less back where he most fully feels at home: in front of a chess board. He used to be The Dark Horse, rolling through opponents with a speed matched only by the trash-talking that came from his mouth. Now he is a shell of that former player, his mind having betrayed him.

Cliff Curtis plays Genesis Potini in the best performance of his career, after doing the rounds in Hollywood and establishing himself as a capable character actor. But he has never so completely transformed into another person like this. What strikes us initially is how he has physically grown into the role, no longer the wiry punk that he usually portrays (a stick next to the hulking beasts in Once Were Warriors). Curtis doesn't merely wear Potini's real-life poncho, he lives inside its faded folds. He has the shaved, misshapen head, the rotten teeth, and most vitally, the tired eyes that never quite disappear even when he is playing the game he loves. His mental illness is always at bay, ready to rear its ugly head - Curtis fills the troubled role with ticks and fidgets and back-of-the-mind mutterings, and in his place another actor might have overplayed their hand, going the route of external flashiness. But Curtis imbues Potini with such trembling, nervous energy that we are amazed he hasn't yet dropped dead from exhaustion and weariness. We see how he channels it through his passion for chess, and how crestfallen he is when his bipolar disorder lets him down at a crucial moment.

Observe how these ticks work in the opening scene, where Curtis caresses and greets chess pieces in a dusty pawn shop as if they were old friends, whispering in their ears like his illness does to himself. There is no high and low in Potini's mental struggle, but a constant uphill battle that might be momentarily eased by a simple game. Away from the chess board, James Napier Robertson wields the soundtrack like an assault on Potini's mind, layered with incessant voices, initially the thousands of commands and medical orders from the numerous doctors he has been treated by, and then finally his own demons taunting him for his personal failures. Retreating within, he mindlessly mumbles coping mechanisms and strategies borne out of a weary existence. The script offers him tiny, brief windows of lucidity where our hearts break for the man crumbling inside; a teary plea to his older brother, a desperate appeal to a teenage about to be led astray.

Like the great New Zealand film by Lee Tamahori, The Dark Horse offers another sobering insight into the lives of a disenfranchised Maori community, filmed in dejected sections of Gisborne where boys are expected to become criminals. The film doesn't seek to pull punches - there are times where we might be inclined to bitterly agree with Ariki as he lays out how the world has forgotten about them, and how a silly little kid's game will do nothing to help them. At times the film might lean towards that foolish naivety, as it builds towards that expected resolution of the underdog triumphing in impossible circumstances. But to watch Potini play chess is to witness a broken, addled mind suddenly whir to life, and we can see how this silly little kid's game has held him through many tough periods.

Ultimately The Dark Horse isn't totally about Mana and the rest of the Maori youth succeeding beyond their situation. Criticisms aimed at the formulaic competition miss the point - win or lose, each of these kids have a long battle ahead of them, and this was merely a waypoint towards a long-term goal. Chess, as Potini outlines, is one way of uncovering their cultural pride and identity; the queen as the Earth Mother, the king as Maui, leader of his people. But every piece is a warrior in their own right, and whether you're a pawn or a king or a queen or a bishop, everyone is welcome to play on Genesis Potini's board. What we eventually realise is that while chess was his way of saving these kids, it was also the key to saving himself.