The episode starts with District Attorney (DA) Harvey Dent dreaming of a confrontation with a mysterious doppelganger, and frightened by his twin's repetitive flipping of a two-headed coin. The dream ends just as the dark, ominous double assures Dent: "It's time, Harvey...it's time!" Dent awakens in his office and heads out to meet with Commissioner Gordon as he leads an assault on a derelict building that has been taken over by members of Rupert Thorne's gang. Batman intervenes, and their combined efforts ensure the gang's capture. Dent publicly congratulates the police and proudly thanks Gordon just as one of the captured crooks splashes Dent's suit with mud, taunting him with the assurance that he and his comrades will be free even if they are convicted. In a fit of rage, Dent attacks the crook, eventually having to be restrained. A second later, Dent inexplicably reverts back to his amiable, courteous self, with seemingly no memory of what has just happened.
Meanwhile, Thorne grits his teeth as Dent's war on crime cripples his criminal empire. He orders his moll, Candice, to dig up the DA's secrets. Bruce Wayne, meanwhile, becomes concerned about Dent's mental health when Dent threatens his secretary after learning Thorne's men have been released on "lack of evidence" (thanks to bribes paid by Thorne himself). As one of Dent's oldest friends, Wayne advises him to find psychiatric help. Since Dent is in the middle of a reelection campaign as Gotham's DA, however, and so has to keep his problems secret to avoid bad press. He takes Bruce's advice, however, and goes to Dr. Nora Crest for treatment.
While under hypnosis, an alternate personality known as "Big Bad Harv" surfaces. This new personality is as evil as Dent is noble. After a violent confrontation in Crest's office, Crest pleads with Dent to remain interned, but he refuses, instead accepting therapy during the reelection race. Unbeknownst to him, however, Candace has been listening at the door, and recorded the session.
Later, Dent gives a speech with a crowd of his supporters, Bruce Wayne, and his fiancée, Grace Lamont, present. During the ensuing party, however, Dent receives a call from Thorne in a secluded office, who warns him to board a limousine waiting for him outside the manor - or risk the exposure of "Big Bad Harv."
Dent leaves, bumping into Wayne on his way out. After noticing his friend's odd demeanor, Wayne decides to investigate (as Batman). He follows the limousine to a desolate chemical plant, where Thorne and his gang confront Dent, threatening to expose his treatment records to the press unless the DA complies with his demands. The file reveals that, as a child, Dent fought back against a bully, only to learn the bully had gone into the hospital (albeit for appendicitis) and had repressed his anger from that point on, thus creating "Big Bad Harv." Pushed to the brink, Dent assumes his alternate personality and attacks Thorne and his accomplices, wiping out the gang with Batman's reluctant help. He then sees Thorne seize the therapy records, and chases him through a walkway of the plant. A thug tries to shoot Dent, but Batman slams into him, causing the thug to shoot instead a vital switchboard, the cables of which touch the intensely volatile compounds lying in the vat below Dent. The ensuing explosion nearly kills him, and Batman's reaction to his unseen face implies that he has been severely disfigured.
As Dent awakens and his bandages are removed, he demands a mirror, just as Grace walks down the hall to meet him - and sees him screaming and bolting from his room. To her horror, she sees that the left side of his body is hideously disfigured, and faints. Dent mutters a rueful farewell, and flees through the hospital's window.