PLOT

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 (led by Kyle Reese (Jai Courtney) and himself) 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 a cryptic warning of events in the year 2017 at San Francisco. Moments before Kyle disappears, he witnesses one of the Resistance soldiers attacking John.

The T-800 arrives in 1984 Los Angeles, but is quickly killed by Sarah and "The Guardian" (Arnold Schwarzenegger), a reprogrammed T-800 (Model 101). Shortly after, Kyle is rescued from a T-1000 (Lee Byung-hun) by Sarah and the Guardian. Kyle learns that the original timeline has been altered: Sarah already knows about Skynet, Judgment Day, and Reese's role as her protector. After the T-1000 tracks the trio, they manage to lure it to a sewer where Sarah disintegrates it with acid.

Sarah then explains to Kyle that the Guardian was sent to protect her from the T-1000 back in 1973, where the Terminators' arrival altered history. After saving Sarah, the Guardian began training her to face her destiny. The Guardian has built a time machine similar to Skynet's and Sarah plans to use it to travel to 1997 to change the events that would lead to Skynet's initial attack on humanity. However, Kyle, convinced that the future has changed because of the warning he received, persuades Sarah and the Guardian that they should travel to 2017 instead. He believes that Skynet will begin its attack on that year of the altered timeline; Kyle and Sarah time-travel while their Guardian spends the next 33 years preparing for their arrival.

In 2017, Kyle and Sarah materialize in the middle of a busy highway at San Francisco and are apprehended by the city's police. The Guardian watches from a distance and follows them to a hospital. While being treated for injuries, Sarah and Kyle learn about "Genisys", a soon-to-be-unveiled operating system that will be used on every computer system worldwide: they realize that it is Skynet's precursor. She and Kyle are then rescued by their son, John Connor. As he leads the pair to a parking garage, the Guardian appears and shoots John, who revives himself, revealing himself to be a nanomachine hybrid. John explains that shortly after Kyle was sent back, he was infected by a T-5000 (Matt Smith), and was subsequently transformed. His mission here is to ensure that the Cyberdyne Systems Corporation would receive Skynet's technology and secure its rise. John, now the T-3000, tries to convince Sarah and Kyle to join the machines and end the conflict, but they refuse. After a brief battle, the T-3000 is temporarily incapacitated with an MRI machine.

The Guardian takes Kyle and Sarah to a safe house near the Golden Gate Bridge, where they make final preparations to destroy the Cyberdyne's Genisys mainframe before it comes online. The T-3000 pursues them to the Golden Gate Bridge, where Kyle, Sarah, and the Guardian are all taken into police custody. While awaiting interrogation, the trio is freed by Detective O'Brien (J. K. Simmons), who discovers that Kyle and Sarah are time-travelers. He thanks them for saving his life from the T-1000's attack back in 1984. The three hijack a helicopter on the rooftop and head toward the Cyberdyne's headquarters, with the T-3000 in close pursuit. During the airborne chase, the Guardian dive bombs into the T-3000's helicopter, causing it to crash.

Emerging from the helicopter crash, the T-3000 enters the Cyberdyne complex and advances the countdown mechanism from 13 hours to 15 minutes. As Genisys begins to gain sentience, the Guardian, Kyle and Sarah plant bombs at key points in the facility while holding off the T-3000. After a lengthy battle, the Guardian traps the T-3000 in the magnetic field of the prototype time machine and destroys it, but not before the T-3000 manages to throw the Guardian into a vat of mimetic polyalloy liquid just beneath the magnetic field. Kyle and Sarah manage to reach a bunker beneath the facility moments before the time machine explodes, setting off the bombs, and preventing Genisys from coming online. The Guardian, now upgraded with mimetic polyalloy components, appears and helps them find a way out of the debris.

Later, the trio travel to young Kyle's (Bryant Prince) home, where they repeat the warning about Genisys to young Kyle, ensuring the events leading to their arrival. Sarah is relieved believing that she is now free to choose what to do with her future. She decides to stay with Kyle.

In a post-credits scene, it is revealed that the Genisys system core was located in a hidden subterranean chamber and has survived the explosion