The story recounts the beginning of Bruce Wayne's career as Batman and Jim Gordon's with the Gotham City Police Department.
Bruce Wayne, aged 25, returns home from training abroad in martial arts, manhunting and science for nearly twelve years. In Gotham, he bides his time, waiting for the right moment to strike, all the while preparing himself. Gordon, meanwhile, has moved to Gotham from Chicago with his pregnant wife, Barbara Kean-Gordon, and pursues a career in law enforcement. His first time out patrolling reveals to him the disturbing nature of law enforcement in Gotham as a senior officer, Detective Flass, assaults an unsuspecting teenager for "staying out late". Gordon is disgusted with his partner's behavior towards all the "offenders" Jim feels that he has to straighten things out.
Bruce makes preparations - registering at a hotel to provide an alibi, giving himself a fake scar to disguise himself - before going out for his first street mission. He enters the Red Light District of Gotham. A young prostitute named Holly Robinson tries to proposition him. Her pimp, angry because he knows Bruce isn't the type to hire prostitutes, forcefully drags her away. Bruce confronts him and gets into a fight, and a few others join in. Selina Kyle, a dominatrix in the slums of Gotham, jumps from her window and fights with Bruce; he is stabbed in the thigh by Holly.
The police arrive on the scene, shoot Bruce and then throw him into the back seat of their car. On the way to the station, he manages to escape by causing the corrupt officers to wreck their squad car. After pulling the unconscious officers out of the car Bruce returns home, bleeding from his various wounds. There he sits, looking for inspiration, something he feels will strike fear into the hearts of criminals. A bat crashes into the room through a window and perches on a sculpture of his father, to which Bruce immediately responds. He has found what he is looking for, stating the words "Yes Father, I will become a bat."
Gordon tries to clean up GCPD, but on the orders of the corrupt Commissioner Gillian B. Loeb, is attacked and threatened by fellow officers. After recovering, he visits the house of one of these officers, where they have gathered to play poker. He waits for Detective Flass, who he knows is responsible for organizing the attack. Flass is the last to leave, and Gordon tails him into the woods, where he proceeds to attack the drunken officer.
Bruce goes out for the first time as Batman and stops a trio of teenagers from stealing a television. A brief struggle ensues, resulting in the Dark Knight's first victory. The legend quickly grows with Batman attacking criminals with increasing boldness, including Flass, who was present at one crime, receiving a pay off from the criminals. One night, when the corrupt city leaders and gangsters like Carmine Falcone gather for a dinner party, Commissioner Loeb explains why Batman is politically advantageous to themselves, assuming he would never bother them; meanwhile, Batman sneaks onto the grounds, puts the guards to sleep and sets up stage lights around the window that comprises one of the dining room walls. He cuts the electricity, throwing the room in darkness, blows a hole in the outside wall and then activates the lights. He gives the men and women a dire warning that he is just as determined to deliver them to justice as well, then leaves. Meanwhile, Selina Kyle is inspired to become a costumed cat burglar when she sees Batman in action and becomes Catwoman.
The police try to capture Batman numerous times, but Bruce is too elusive and alert to fall for their traps. In addition, the maverick district attorney, Harvey Dent, becomes Batman's secret ally. After a night of following useless leads, Gordon and his partner, Detective Sarah Essen, see a truck barreling down the street. They give chase and Gordon hands the wheel over to Essen as he tries to get into the vehicle. An old, homeless woman stands in the way of the truck and is about to be run over just as soon as Batman jumps in and shoves her out of the way. The bus runs into a wall and Gordon briefly blacks out, only to awake moments later and find Essen holding Batman at gunpoint. She is momentarily distracted when she turns to ask if he is all right and Batman takes advantage to disarm her and flee into an abandoned building.
When cops arrive on the scene, the commissioner is quick to call in the trigger-happy Branden and his squad to drop a bomb on the building, which the Commissioner claims has already been scheduled for demolition. While dodging the fire from the explosion, Batman's belt (which contained explosives) catches fire, and he is forced to discard it. After suffering two dizzying gun wounds, Batman escapes into the secure basement and survives the blast. A crowd gathers outside the building. Stuck with only a blow gun and 3 darts, Batman uses a small device in his boot to summon thousands of bats from his cave to the building. A battle occurs as the police storm into the building and hunt him down. He incapacitates some and even saves a cat, jumping out of the building (after throwing a police officer forcefully through a wall) and takes advantage of the chaos that occurs when the bat colony arrives to speed away on a police motorcycle and escape.
Gordon has a brief affair with Essen. During the affair, he is confronted by the Commissioner, who threatens to inform his wife of the affair if he doesn't comply. Gordon, after bringing his wife to an interview with Bruce Wayne, whom he and others suspect of being Batman, stops the car in the driveway on the way back and tells her about his affair. Essen later leaves for New York.
Months pass and Batman overhears a local mafia boss planning revenge against Gordon. Selina Kyle, frustrated because she feels her petty crimes aren't enough, interferes and attacks the group. Batman does not appear, but helps Selina from the shadows, throwing small bat-shaped blades laced with tranquilizers at some of the men. Bruce, while working out, figures out the plan based on the part of the conversation he was able to record.
Gordon is called away by the police to investigate a robbery. On his way out, a mysterious motorist entering his garage raises Gordon's suspicions, as Gordon has never seen the motorist. He returns to the garage only to find his wife and baby being pulled into a car. He shoots and kills the men trying to take his wife, who survives; however, one assailant is unharmed. The car leaves with Gordon's baby in it, and Gordon shoots the motorist, takes his motorcycle and follows. The motorist, Bruce, is unharmed thanks to a bullet-proof vest. He attempts to leave, but not before Barbara threatens to shoot him. She lets him go when he promises to save her baby, takes a bicycle from a passing stranger, and pursues Gordon and the car.
Gordon shoots out a wheel on the car and it crashes into the side of a bridge. The don's hired knife, his nephew, exits the car, baby in hand. A struggle ensues and the baby is thrown off the bridge, followed by Gordon. However, Bruce had already arrived and dived after the baby before Gordon even falls over the rail. Gordon, having lost his glasses in the struggle with the hitman, thanks Bruce (whom he claims to not recognize due to his aforementioned missing glasses) and makes it clear that he won't turn him in.
Dent and Batman's efforts bear fruit with Flass, who is persuaded to turn damaging states evidence against his superiors, including a disgraced Commissioner Loeb, who is forced to resign. Although his immediate replacement, Grogan, is apparently worse, Gordon is content for the moment with receiving a job promotion and family counseling with his wife. The story ends with the new Captain Gordon waiting on the rooftop of the GCPD headquarters for Batman, to discuss somebody called The Joker and his scheme to poison the reservoir.
The story also includes the first appearance of Mafia don Carmine Falcone.
It has been announced that there will be an animated movie made.
There is an adaptation of Frank Miller's Batman Year One story that is being turned into a DC Animated Movie.