"The Dark Horse" is based on the true story of Genesis 'Gen' Potini (Cliff Curtis), a Maori speed-chess champion seeking redemption and a new purpose in life despite his struggles with bipolar disorder. A former chess prodigy, Gen is brilliant and charismatic, bringing unusual, potent energy to a game most often played with quiet reserve.

Upon his release from an institution, he is remanded into the custody of his older brother Ariki (Wayne Hapi), the leader of a rough street gang planning the initiation of Gen's reluctant teenage nephew Mana (James Rolleston).

When Gen volunteers to coach the ragtag young members of the Eastern Knights chess club, Mana is inspired by his uncle's determination to bring hope to the children of the club and turn his troubled life around, while seeing it as a chance to possibly save his own.