During a conference of crime bosses held in a Gotham City skyscraper, gangster Chuckie Sol is killed by a mysterious cloaked figure, shortly after Batman bursts in on the meeting. Due to the killer’s resemblance to Batman, the Dark Knight is blamed for Sol’s death. Councilman Arthur Reeves tells the media that Batman is a public menace (despite Commissioner Gordon’s protests), then later attends a party at the mansion of billionaire Bruce Wayne, Batman’s secret identity. Reeves teases Bruce about his bad luck with women and for having allowed an old girlfriend, Andrea Beaumont, to get away.
In a flashback to 10 years before, Bruce meets Andrea in a cemetery while visiting his parents’ grave. That night, in one of his first crime-fighting attempts, Bruce foils an armored car robbery while disguised in a black ski-mask and leather jacket. Though he succeeds, he is discouraged that the criminals did not fear him. Around the same time, he begins a romance with Andrea. Eventually, Bruce decides to abandon his plan to become a crime-fighting vigilante and proposes marriage to Andrea. Soon afterward, however, Andrea mysteriously leaves Gotham with her father, Carl Beaumont, ending her engagement to Bruce in a Dear John letter. Believing that he has lost his last chance of having a normal life, Bruce dons the mask of Batman for the first time.
The mysterious killer finds and murders another gangster, Buzz Bronski. Around the same time, Batman discovers that Andrea has returned to Gotham for the first time in 10 years, and she ends up finding out that Bruce is Batman. Batman soon finds evidence linking Andrea’s father with gangster Salvatore Valestra, for whom both Sol and Bronski once worked as enforcers. When he visits Andrea to try to get more answers, she rebuffs him over the choices that he made while she was away. The killer later targets Valestra, who turns to the Joker for help. The killer arrives at Valestra’s house, and finds the gangster already dead at the Joker’s hands; the house explodes, with the killer barely escaping. Batman pursues the killer, but is interrupted by the police, who try to arrest Batman. Andrea rescues Batman in her car, and they spend the night together at Wayne Manor. Andrea explains to Bruce that she and her father had left Gotham and had been hiding in Europe from the Valestra mob, to whom he owed a lot of money. Batman comes to suspect that Andrea’s father may be the killer, but later gets Reeves (who was told of Batman’s innocence by the Joker before being poisoned by him, as he believed the Councilman to be the killer) to confess that he told the Valestra mob where Beaumont was hiding in return for campaign contributions, and that the mob ordered Beaumont’s death.
The killer tracks the Joker to his hideout – an abandoned world’s fair amusement park – and removes its ominous costume: the killer is Andrea, intent on avenging her father’s death at the hands of the Joker, who is revealed to be the last surviving member and professional hitman of the Valestra mob. Having already deduced her identity, and ready for her attack, the Joker fights her. Just before he can kill Andrea, Batman arrives and saves her from the Joker, and begs Andrea to give up her quest for revenge. She refuses, stating that the mob ruined her life by taking away her future with him; she tells Batman that he himself is driven by revenge before disappearing. Batman battles with the Joker, a struggle that ends in a stalemate. Moments later, Andrea returns and seizes the Joker, bidding Batman goodbye before vanishing with the maniacally laughing clown in a cloud of smoke as the entire amusement park erupts in a series of rigged explosions. Batman barely escapes by falling into a waterway and being swept away to safety by the current.
Alfred later consoles a heartbroken Bruce, telling him that no one could have helped Andrea. Bruce finds a locket containing a picture of himself and Andrea left behind in the Batcave. Meanwhile, Andrea is shown standing alone on the deck of a departing ocean liner. In the final scene, Batman stands alone on the top of a Gotham building; when the Bat-Signal appears in the sky, he swings off into the night to continue his war on crime.