* Christian Bale as John Preston
* Sean Bean as Errol Partridge
* Emily Watson as Mary O'Brien
* Taye Diggs as Andrew Brandt
* Angus Macfadyen as Vice-Counsel DuPont
* Sean Pertwee as Father
* William Fichtner as Jurgen
* Emily Siewert as Lisa Preston
* Matthew Harbour as Robbie Preston
* Alexa Summer and Maria Pia Calzone as Viviana Preston
* Dominic Purcell as Seamus
* Brian Conley as Reading Room Overseer
* David Hemmings as Father's Chamberlain
Equilibrium is set in the futuristic, and dystopian city-state of Libria. In the year 2072 , the Third World War devastated the planet. Realizing that humanity likely wouldn't survive another war, the leaders of the world sought to create a society free of conflict. It was determined that human emotion was the primary cause of conflict, and thus any and all emotionally stimulating material was banned. These materials are rated "EC-10" for "emotional content" (a reference to the MPAA film rating system), and are typically destroyed by immediate incineration. Furthermore, all citizens of Libria are required to take regular injections, called "intervals," of an emotion-suppressing drug called Prozium, collected at the distribution centers known as "Equilibrium".
Libria is governed by the Tetragrammaton Council, which is led by a reclusive figurehead known as "Father". Father never interacts with anyone outside the ruling council, but his image is omnipresent throughout the city in a strong cult of personality. The Tetragrammaton Council strives to create identical lives for all Librians and uses its police state apparatus to enforce unity and conformity. At the pinnacle of Librian law enforcement are the Grammaton Clerics, who are trained in the deadly martial art of Gun Kata, an art which teaches users to predict the actions of opponents during firearm combat. The Clerics exist for the purpose of locating and destroying EC-10 materials and for pursuing, apprehending, and, if necessary, terminating "sense-offenders"—people guilty of feeling emotions.
Despite the efforts of the police and Clerics, a resistance movement exists in Libria, known as "The Underground". Members of this movement are responsible for terrorist activity against Libria, specifically against the Prozium factories. The leaders of the Underground believe that if they can disrupt the production and distribution of Prozium, the emotionally-awakened Librians will rise up and destroy the Tetragrammaton Council. The Underground operates within Libria itself, but also has contact with resistance groups residing in the "Nethers", the ruins of cities destroyed during World War III. These outsiders hoard objects and artifacts from the old society before World War III, including art and literature. Subsequently, they are the targets of Librian death squads composed of police and Clerics.
The film's protagonist, Grammaton Cleric First Class John Preston, is Libria's highest ranking cleric. He is a widower whose wife, Viviana, was executed after being revealed to be a sense offender, leaving him with two children. After a raid on a group of resistance members in the Nethers, Preston notices that his partner, Grammaton Cleric First Class Errol Partridge, has personally taken a copy of the poems of Yeats under false pretenses. Preston discovers that Partridge has not turned the book over for destruction and follows him to a ruined cathedral in the Nethers, where Partridge speaks of emotion and forces Preston to aid him in suicide by cop. Shortly afterward, Preston accidentally breaks the vial of his morning dose of Prozium and, unable to obtain a replacement due to terrorism closing the equilibrium center, begins to experience emotions.
Preston is assigned a new partner, the career-conscious Brandt. Following the arrest of Mary O'Brien, who has stopped taking Prozium, his emotional confusion is exacerbated during her interrogation. He first acts out of emotion when he makes an excuse not to execute a puppy in the Nethers. Preston has by now ceased taking Prozium and is forced to try and maintain his monotone and emotionless facade in front of his son and the increasingly suspicious Brandt. Over the course of the film, Preston's behavior increasingly mirrors that of Partridge in the beginning, even to the point of repeated dialogue.