In the brief prologue it's explained that in America, all the population lives within a huge city, Mega City, which spans from Washington DC to Boston. Outside the walls is the "Cursed Earth", a barren wasteland. The population is so large, 850 million, that the law is carried out by Judges, who act as judge, jury and executioners. Dredd (Karl Urban of Star Trek) is one such Judge. He's assigned a new partner, Anderson (Olivia Thirlby of Darkest Hour), who has strong psychic powers. Although she barely passed her training his boss thinks she might be an advantage to law enforcement, so it's his job to take her out for her first day. She asks what they are going to do first and he tells her to take her pick since they can only respond to 6% of the crimes that happen in the city at any given time. They go to a huge building called Peachtrees, inhabited by 75,000 tenants, where three men were thrown off a top floor. They had pissed off the local crime lord, Ma-Ma (Lena Headey of Game of Thrones, Sarah Connor Chronicles). She's vicious--a scarred ex-hooker who has attained power. Once Dredd and Anderson are inside the structure she closes all the doors and windows and announces to anyone that if they kill the Judges they'll be rewarded. Likewise, if they help them they'll be slaughtered. So Dredd and his new partner have to not only survive being hunted by hundreds but get to Ma-Ma and carry out her sentence.
Incredibly violent, far more faithful to the comic book source material than Sylvester Stallone's Judge Dredd (1995). I also liked that they never try to "humanize" Dredd and he keeps his helmet on throughout the entire movie. Highly recommended.