A scrolling and narrated prologue opens the film: in the Third Millennium (the 21st Century), the Earth's ecosystem has become polluted and altered, leaving much of the world's landmass a barren desert called the Cursed Earth. Most people have moved to a few "mega" cities, where, due to the rampant population increase, law and order have collapsed. A new order has risen, that of the Street Judges, who are "judge, jury and executioner", possessing the license to fight criminals, pronounce sentences and summarily execute them if necessary.
When the story opens, Herman "Fergee" Ferguson is returning to Mega City 1 after serving a sentence in the Aspen Penal Colony. He arrives at his new home, Heavenly Haven, a dangerous neighborhood which is embroiled in a vicious "block" war. Gangsters have set up their command post in his new apartment and have opened fire on their rivals in another building and on the street, 40 stories below. Fergee finds himself an unwitting accomplice, passing ammunition to the block warriors.
Street Judges Hershey and her candidate judge respond to the block war. Hershey calls immediately for backup, which arrives in the form of Judge Joseph Dredd, the most famous of all the Street Judges. He leads an assault on the block warriors, dispatching several of them rapidly with his pistol, the Lawgiver 2. Hershey's candidate is killed when he rushes into the command post and is shot immediately. Dredd kills everyone but the block war leader and charges him with several violations of the Mega City Municipal Code, before summarily executing him for the candidate's murder. He announces that court is adjourned. Outside the apartment, he finds Fergee hiding in a servo-droid and finds him guilty as a repeat offender, giving him five more years in Aspen. Fergee claims he's innocent and was hiding from the block warriors, however, Dredd refuses to be swayed and dismisses Fergee to re-incarceration. Dredd reports to Chief Justice Fargo, who questions Dredd's hardened methods of summary execution. He believes Dredd when the judge tells him that the executions were unavoidable. Fargo assigns Dredd to teaching ethics at the academy a few days per week.
At the Aspen Penal Colony, Warden Miller meets with a man named Rico and gives him a small object that contains a copy of Rico's badge and a picture of a reporter, Vartis Hammond, who has done several unfavorable reports on the Street Judges. Rico suddenly unlocks a secret compartment in the box and it becomes a small pistol, which he uses to shoot Miller through the throat. Miller is unable to match the voice-identification that disarms the cannons in Rico's cell and he is immediately killed by them. Rico hides and is able to massacre the guards who rush into the room.
At the Academy, Dredd tells a pool of cadets about the weaponry of a street judge and how little of it matters where knowledge of the Mega City Municipal Code is concerned. He also tells them that they stand a high chance of being killed on duty and that, if they live to old age, will undertake the Long Walk into the Cursed Earth, where they will teach the savage denizens outside the city walls of the law. Hershey later speaks to Dredd about his obstinate personality and whether he had any friends. Dredd tells her that he did have a friend at the Academy but was forced to judge and sentence him when he broke the law.
Rico stows away on a shuttle from Aspen in a body bag. In Mega City, he goes to a pawn shop and retrieves a box set aside for him. In the box is a judge's uniform and a Lawgiver pistol. When Rico reaches for the pistol, the pawnshop owner warns him not to pick it up because all Lawgiver's are booby-trapped and will only recognize the grip of a street judge. Rico picks up the pistol, unharmed by the defense mechanism, and shoots the man dead, revealing that he is a judge himself. Rico finds a giant combat droid, an ABC Warrior, in the shop and reactivates it, giving it the duty of being his personal bodyguard and a mission of "war".
At Hammond's apartment, an unidentifiable judge bursts in and kills Hammond and his wife. The incident is captured on a security camera and Hammond's final word is "DREDD!!" Dredd is arrested on the street and incarcerated for trial.
Dredd chooses Hershey to defend him due to her expertise in criminal law. At the trial, the recording from the security camera is presented, however, Hershey successfully has the evidence dismissed as circumstantial since Dredd cannot be positively identified. However, new evidence is presented; the Lawgiver 2 pistol has a secret feature that captures the DNA of the wielder and imprints it on every round of ammunition fired. The DNA from the rounds that killed Hammond and his wife are an exact match for Dredd himself. Hershey and Dredd's defense crumbles and Dredd is sentenced to death. Fargo, who is overcome with guilt at seeing Dredd, one of his greatest students, sentenced, decides, with some goading from Justice Griffin, to retire as Chief Justice and leave Mega City on his long walk into the Cursed Earth. Before he resigns, Fargo asks the Justice Council to commute Dredd's death sentence and send him to the Aspen Penal Colony for life. The council agrees, Fargo leaves Mega City in honor and Griffin becomes Chief Justice.
Later, when Griffin returns to his office, he is confronted by Rico, who demands access to a project called "Janus". Griffin tells Rico that there must be a demand for the project to be implemented; violent crime must engulf the city for Griffin to convince the Justice Council to re-institute Janus. Rico promises to do just that.
Dredd is put on to a shuttle going to Aspen; seated next to him is Fergee. (Seated around them are several criminals who were sentenced by Dredd himself.) Dredd and Fergee argue briefly about how Dredd unfairly re-convicted Fergee; Dredd claims "the law doesn't make mistakes" and Fergee asks Dredd to explain how the judge became convicted himself. Just then, another con on the shuttle attacks Dredd. On the ground, the Angel Family, religious fanatics and pirates of the Cursed Earth, blast the shuttle out of the sky with a rocket launcher. In the ensuing chaos, the pilots of the shuttle are killed. Dredd and Fergee are the only survivors and are captured by the Angels. In the Angel Family's lair, Dredd identifies them as pirates and scavengers. Fergee, in an attempt to escape their clutches, suddenly proclaims himself a believer in the Angel's religion and is let go. Dredd then reveals the Angel's darkest secret; they are cannibals and plan to eat Fergee in sacrifice. Dredd frees himself and is immediately attacked by the Angels, whom he takes out one at a time. When Pa Angel attacks Dredd, he is shot dead by one of the salvage team sent by Chief Justice Griffin himself to investigate the shuttle crash nearby. The two salvage men prepare to execute Dredd when they are shot dead by Chief Justice Fargo, who'd found the crash site. Fargo himself is critically wounded by Mean Machine Angel, whom Dredd had knocked out moments before. Dredd fights Angel and electrocutes him. As Fargo dies from his wounds he tells Dredd about Janus, a sophisticated genetics project that tried to clone children with the best traits of the existing judges in an attempt to create the perfect Judge. There were two subjects produced, Rico and Dredd himself, who never knew Rico was his brother. The project was a massive failure with Rico becoming a psychotic monster, judged by Dredd and sentenced to Aspen for life. Fargo dies and Dredd vows to return to Mega City and stop Rico from starting the Janus project again.
Back in Mega City, chaos erupts as Rico, using his ABC robot, declares war on the Street Judges. Many are killed and their numbers reach a critical point. Hershey detects a power surge near the Statue of Liberty and moments later is almost killed when her Lawmaster cycle explodes. The Judges Council meets and discusses the crisis, knowing that they cannot replenish their ranks quickly. Griffin suggests that they unlock the Janus project, which can produce a fully mature judge in mere hours (with the technology being more advanced than 20 years before, it's possible to create adult clones rather than children). Each member of the council gives their authorization, as does Griffin, however they collectively object when they realize that the project was already a major failure. Just then, Rico enters the room and murders the entire council, leaving Griffin alive.
Outside the city walls, Dredd and Fergee are able to enter the city through a heat vent, barely making it inside. They enter the Halls of Justice, where Dredd steals a judge's uniform. Dredd finds the murdered members of the council and Griffin. He also sees Rico but fails to stop him. Griffin shoots himself in the arm, framing Dredd. When guards move in to arrest Dredd, he flees on a newer model Lawmaster, flying off with Fergee. After a furious chase, they escape.
Dredd finds Hershey at her apartment and tells her of Rico's plan. She goes with him to the Statue of Liberty where Rico has already started the Janus project. When Griffin objects to Rico replacing the old DNA samples with his own, Rico has the ABC robot dismember him alive. When Dredd, Hershey and Fergee try to enter the lab, Rico has the robot shoot Fergee and capture Hershey. With Rico threatening to have Hershey killed, Dredd agrees to listen to Rico explain his plan to create a super race of judges that will do only their bidding. Dredd naturally refuses to join Rico and they battle hand-to-hand. Fergee distracts the ABC robot long enough to disable it and Hershey fights with Rico's assistant, Ilsa Hayden, chief scientist of the Janus project. Rico finally corners Dredd where he hangs above the city. Rico tells Dredd that his judgement is death for betraying Rico years before. Dredd is able to distract Rico with a flare from his Lawgiver and pulls Rico over the edge where he falls to his death. Hershey finds Dredd and helps him to safety.
On the street, Dredd is greeted as a hero. When asked if he'd like to take the position of Chief Justice, he refuses, saying he would rather remain a Street Judge.