In the year 2029, John Connor (Jason Clarke), leader of the Resistance against Skynet launches a massive final offensive on Los Angeles. He is notified by his unit, Tech-Com, that Skynet will attack on two fronts past and future, thereby changing warfare forever. Connor sets up two attack forces, one to strike at Skynet's main defense grid, and a second one, led by John and his trusted lieutenant Kyle Reese (Jai Courtney), to destroy Skynet's main weapon, a time machine hidden at a remote storage facility.
Before the Resistance can reach the machine, Skynet sends back a T-800, to the year 1984 to kill John's mother, Sarah Connor (Emilia Clarke). Learning of this, Reese volunteers to go back in time in order to protect her and ensure John's existence. As Kyle floats in the machine's magnetic field, he receives fragments of memories belonging to his alternate 12-year-old self (Bryant Prince) with young Sarah, including a cryptic warning of events in the year 2017. Moments before Kyle disappears, he witnesses one of the Resistance soldiers attacking John.
The original T-800 arrives in 1984 Los Angeles, where it confronts three punks and demands their clothes, as before in the first film. All is usual as it happened in the first film, when suddenly a new, much more aged T-800 (Arnold Schwarzenegger) appears to confront the original. The three punks run away just as the two cyborgs engage in a one on one brawl, with the younger one gaining the upper hand. However, Sarah arrives just in time; saving the new T-800 by shooting the original T-800 before it can finish his rival. Shortly after, Kyle is chased by a police officer, taking his pistol and demanding the cop tell him the date and year as before in the first film. Unfortunately for Kyle, the cop is actually the T-1000 (Lee Byung-hun), sent back in time by Skynet to kill Kyle.
Kyle flees before getting detained by two real police officers in the clothes store. The T-1000 kills one of them before attacking the other. Sarah then arrives to save Kyle while holding off the T-1000, who then pursues the duo, but they manage to escape. Kyle finds out that the original timeline has been altered: Sarah already knows about Skynet, Judgment Day, and Reese's role as her protector. More of which, Kyle learns that the T-800 was already dispatched by Sarah and the new Terminator, who is now known as "The Guardian" (or as Sarah sometimes calls him, "Pops"). After a brief argument between Kyle and The Guardian, who loads Kyle into the back of the van with Sarah while taking the wheel, the T-1000 attacks the trio, who plans to lure the T-1000 into a trap from a hidden building. From there, the T-1000 disables The Guardian and re-activates the fallen T-800. The T-1000 then pursues Sarah while Kyle fends off the T-800. After destroying its flesh, Kyle finally destroys the T-800 by blasting its face off. Later on, two Kyles approach Sarah, one of which is the T-1000, whom she shoots. Sarah then disintegrates the T-1000 with acid before The Guardian finishes him off for good.