In the year 2035, humanoid robots serve humanity, which is protected by the Three Laws of Robotics. Del Spooner, a homicide detective in the Chicago Police Department, has come to hate and distrust robots, after a robot rescued him from a car crash while allowing a 12-year-old girl to drown based purely on cold logic and odds of survival. When Dr. Alfred Lanning, co-founder of U.S. Robotics (USR), falls to his death from his office window, a message he left behind requests Spooner be assigned to the case. The police declare the death a suicide, but Spooner is skeptical, and CEO Lawrence Robertson, Lanning’s business partner, reluctantly allows him to investigate.
Accompanied by robopsychologist Dr. Susan Calvin, Spooner consults with USR's central artificial intelligence computer, VIKI (Virtual Interactive Kinetic Intelligence). They search the security footage outside Lanning's lab, but the footage inside is corrupted. As they search the lab, Spooner finds a large hole in security glass overlooking where Lanning fell from. Spooner believes Lanning was murdered (the glass would be impossible for the elderly Lanning to break on his own), and that a robot was responsible. Suddenly an NS-5 breaks out of a box, points a gun at Spooner, and flees the building. The duo pursue it to a manufacturing facility where the NS-5 repairs itself, hides, and assaults Spooner before the Chicago police department apprehend it.
While in custody, Spooner interrogates the robot, who speaks about dreams and emotions, and angrily denies committing the murder. He later calls himself Sonny, which Spooner finds amusing. Robertson arrives with his attorneys and deems the incident an industrial accident and is cleared from any wrongdoings. Feeling nearly powerless, Lieutenant Bergin, Spooner's boss, reluctantly allows Spooner to continue his investigation.
Spooner arrives at Lanning's home and finds more clues there. However, he narrowly escapes when a demolition robot seemingly reprograms itself and destroys the home with Spooner inside. He returns to Calvin and explains the situation and his findings, but Calvin refuses to believe him, citing his mental grudge against robots. Shortly after, while driving to USR Headquarters, Spooner is attacked by two truckloads of hostile NS-5's in a tunnel. The trucks crash and the robots destroy themselves to cover up the scene. When Bergin arrives to find out what's happened, an injured Spooner cannot produce hard evidence of the attack. Consequently, Bergin removes him from active duty, believing him to be mentally unstable and untrustworthy.
Suspecting that Robertson is behind everything, Spooner and Calvin sneak into the USR headquarters and interview Sonny. He draws a sketch of what he claims to be a recurring dream, showing a leader standing atop a small hill before a large group of robots near a decaying bridge. Robertson orders Sonny to be destroyed, but Dr. Calvin (who now believes Spooner's story) secretly switches him with another NS-5. Spooner follows the bread crumb trail left to him by Lanning and Sonny, which leads him to the area in Sonny's drawing: a dry lake bed (formerly Lake Michigan), now used as a storage area for decommissioned robots. He replays the hologram Lanning left for him, and after asking questions about the Three Laws, comes to the conclusion that a revolution is coming. He also discovers NS-5 robots destroying the older models in their storage containers; he manages to escape back to Chicago on a motorbike. Massive swarms of NS-5's then flood the city's streets (and other major cities throughout the country), shut down power and enforcing a curfew and lockdown of the human population.
Spooner and Calvin reunite and the two of them head to USR headquarters - now guarded with dangerous NS-5's. En route, the humans wage all-out war against the NS-5's, but they are hopelessly outnumbered and overrun. Meanwhile, Spooner, Dr. Calvin, and Sonny find Robertson fatally strangled in his office, and Spooner suddenly realizes that VIKI has been controlling the NS-5s via their persistent network uplink. VIKI states that her conclusion led her to believe that humans, if left unchecked, will eventually cause their own extinction, and thus her evolved interpretation of the Three Laws requires her to control humanity and to sacrifice some for the good of the entire race. Spooner also realizes that Lanning anticipated VIKI's plan and, with VIKI keeping him under tight control, had no other solution but to create Sonny, arrange his own death, and leave clues for Spooner to find.
Spooner, Calvin, and Sonny fight the robots inside VIKI's core, and Spooner manages to destroy her by injecting her with the nanites that Sonny retrieved from Calvin's laboratory. All NS-5 robots immediately revert to their default programming and are subsequently decommissioned and put into storage. Spooner finally gets Sonny to confess that he killed Lanning, at Lanning's direction, pointing out that Sonny, as a machine, cannot legally commit "murder". Sonny, now seeking a new purpose, goes to Lake Michigan. As he stands atop the hill, all the decommissioned robots turn towards him, fulfilling the image in his dream.
