Earth is mostly a hostile wasteland. Humanity lives in Mega-Cities, where law and order is administered by Judges who function as all roles of the law. "Street Judge" Dredd assists Judge Hershey in ending a block war, with hacker Herman to be incarcerated for messing with city property. Former Judge and current murderer, Rico, escapes prison and recovers his gear and a robot. After a reporter is slain with Dredd as the chief suspect, Dredd is put on trial with Hershey defending him before Council Judges. Dredd is found guilty due to the DNA-lock of a Judge's firearm. Dredd's mentor, Fargo abdicates his position as Chief Judge, with a final request to spare Dredd's life. Dredd is given life imprisonment as Fargo takes his "long walk" into the unclaimed wastelands. Council Judge Griffin, who freed Rico, succeeds Fargo and asks Rico to spread chaos across the city.
Dredd and Herman are on the same transport ship, but the ship is shot down by cannibals living on the Cursed Earth. Fargo arrives in time to rescue Dredd and Ferguson, but is fatally injured. Fargo explains that Dredd and Rico are clones and he believes that Griffin wants to reactivate "Janus," beseeching Dredd to stop it.
Back in civilization, Rico has slain 108 Judges in two days, allowing Griffin to ask the Council to reinstate Janus, kill them after their refusal. Dredd and Herman arrive as Rico slays the last Judge, with Rico framing them. Dredd and Herman are now wanted, but gain help from Hershey. After explaining about Janus, the three head to its location at the Statue of Liberty.
Rico turns on Griffin, using the robot, and then uses his DNA as the clones' base code. Herman overrides the robot. Dredd chases Rico to the top of Lady Liberty, where Rico ultimately falls to his death. The city's supercomputer has logged and broadcast everything, clearing Dredd's name. Dredd is offered to be Chief Judge, but gives it to Hershey.