The year is 2161. Genetic alteration has allowed humanity to develop a system where individuals stop aging 25 years after birth. Due to over-population concerns, 'living time' has replaced money as the standard currency and people must acquire more time through labor and commercial means after turning 25 years of age, or die within a year. The amount of living time one has left is represented by a clock embedded on their arms like a wristwatch, and when that clock reaches 13 zeros (0000:00:0:00:00:00), they will die instantly. Each social class lives in a different area called a 'Time Zone'. The poor live in the ghettos of Dayton and work each day to earn a few more hours of life, which they must also use to pay for everyday necessities. The rich live in the luxurious New Greenwich, and can live for centuries to millennia based on how much time they have acquired.
Will Salas (Justin Timberlake) is a 28-year-old factory worker who lives with his 50-year old mother Rachel (Olivia Wilde) in the ghettos. One day, Will and his best friend Borel (Johnny Galecki) visit a bar and encounter 105-year-old Henry Hamilton (Matt Bomer), who has more than a century on his clock and flaunts his time around by purchasing everyone drinks. Hamilton is attacked by Fortis (Alex Pettyfer), the 75-year old mobster boss of a gang called the Minutemen, who are infamous for stealing other people's time by force. Will helps Hamilton escape the confrontation and leads him to shelter at an abandoned factory, where Hamilton tells Will that there's enough time for everyone, but it is being stockpiled for the rich to use in becoming immortal. An upset Will argues that no one should die before their time naturally ends so that others may live, upon which Hamilton describes how he is tired of being alive.
Later that night, Hamilton transfers his remaining time to Will while he is asleep and commits suicide by climbing a bridge as his time expires. Once his time is gone, he falls into the river below. Will arrives too late to save him, realizes he has been filmed by a nearby surveillance camera, and flees the area. Hamilton's body is found by the resident police force, the Timekeepers, who deploy veteran Timekeeper Raymond Leon (Cillian Murphy) to capture Will, who is now believed to have stolen Hamilton's time, then kill him.
Will awaits his mother at a bus station to tell her about his newly gained wealth, only to discover that she didn't have enough time to pay for her usual bus ride after the price suddenly increased. He rushes down the street to find her. They encounter each other on foot, and as she runs and leaps into his arms, her time expires before her son can help her and she dies in his arms. Remembering what Hamilton had told him about the inequity of the time system, Will decides to seek revenge. He visits Borel and gives him a decade's worth of time as a friendship gift, and leaves for New Greenwich with over a century on his clock. Upon arrival, he enters a casino, where he meets a 90-year old millionaire, time-loaning businessman Phillipe Weis (Vincent Kartheiser) and his 27-year-old daughter Sylvia Weis (Amanda Seyfried). Sylvia becomes interested in Will after a tense gambling table showdown where Will beats her father in poker with only seconds to spare on his clock, and she invites Will to a party at her father's mansion.
At the mansion, Will is apprehended by Leon, who confiscates most of his time before Will is able to escape by taking Sylvia hostage. He attempts to return to the ghetto with her; however, his car is ambushed by Fortis, destroying the car in an attack which renders them unconscious. Fortis discovers that Will was in possession of Hamilton's time, but is disappointed to learn that Will has no significant amount of time on him, due to having most of it confiscated by the Timekeepers. As consolation, he steals most of Sylvia's, forced to leave about half an hour on her clock as the approaching Timekeepers scare him away from the scene. Will returns to consciousness and gives Sylvia some of his remaining time so they can return to his old neighborhood. They first visit Borel to retrieve some time Will gave him earlier as their time is running out soon, only to find out from Borel's grief-stricken wife that he had drunk himself to death with 9 years on his clock. Sylvia pawns her jewelry for a meager price of 2 days. Finding themselves shelter later, Will makes a call to Weis demanding a 1,000 years' ransom for Sylvia, to be distributed to the people of the ghetto. Leon traces Will's location from his phone call, and heads to Dayton in pursuit.
The following day, as Will prepares to release Sylvia, he discovers that Weis did not pay the ransom, but Will decides to let Sylvia go regardless. Leon appears and almost kills Will, but is shot in the shoulder by Sylvia. Will then transfers two hours of time to the disarmed Leon so that he is able to walk out of Dayton before he "clocks out". Will and Sylvia escape in Leon's car. Later, Will tells her that she still has a chance to walk away from the situation, but she decides to remain by his side, saying there is no purpose to the life she once had in New Greenwich. The couple then kiss. They begin a series of Time Bank robberies while evading Leon and the Timekeepers, stealing the Time Capsules which store time and distributing them to the poor, with a bounty of 10 years on their heads. Fortis eventually tracks down Will and Sylvia a second time, and challenges Will to a Time Fight. Will dominates the fight by using the technique he learned from his late father and kills the remaining Minutemen while Fortis dies, his time transferred to Will.
However, Will and Sylvia soon realize their previous efforts were futile, as the rich have the ability to simply increase the cost of living in the ghettos to maintain the status quo. They devise a plan to steal a million years from Weis' private headquarters. Will and Sylvia steal Weis' one million year Time Capsule and escape the facility. The Timekeepers form a blockade to stop them, but Will and Sylvia manage to break through and reach Dayton. Upon arrival, Leon crashes his car into Will's, but Will is able to hand the Time Holder to a young girl, who distributes the time among the people. Leon eventually catches up with Will and Sylvia outside the city, holding them at gunpoint. Will jokingly asks Leon to return some of the time he previously loaned him so that they can survive till their executions, but Leon realizes that he had neglected to replenish his own time before going after them, and dies. Will and Sylvia are left with seconds to live, but Will runs to Leon's car and takes his allotted time, and transfers it to Sylvia seconds before she is about to die, a scene mirroring his mother's death.
Will and Sylvia continue robbing banks as part of their efforts to crash the system, now with a bounty of 100 years on their heads, while the rich attempt to cope with the sudden surge of new rich people arriving from the ghettos.