Not long into Broken, single father Sang-Hyun is told by a coworker at his factory, "You have a heart of gold -- that's your problem." His daughter is raped and murdered by a group of teenagers in the next scene and, given his eventual response of tracking down the killers one by one and personally punishing them à la Death Wish, it would appear that his friend is both right and wrong. Sang-Hyun is so self-abnegating and concerned with pleasing others in the early going that his eventual vigilantism represents both a breaking point and a liberation, even a becoming.
The police are dutiful but ultimately ineffectual, as they so often are in movies about aggrieved parents of murder victims, and the gray area in which Lee Jung-ho's film spends most of its time does little to suggest that our (anti)hero is unjustified in his response: There's the letter of the law, and then there's the complex range of visceral reactions to tragedy. Sang-Hyun hunts down his daughter's murderers at the same time the police catch on to his activities, and Lee treats the overlapping searches as both suspenseful developments and a way to more fully understand his protagonist.
Jung Jae-young gives a physical, full-bodied performance in the main role; the transformation Sang-Hyun undergoes will devastate you even before you learn that he's doing this not just out of rage but in order to keep the memory of his daughter alive; if he doesn't do it, his grief-addled mind reasons, then who will?