In 2029, the leader of the human resistance, John Connor, leads the war against the machines. At the Los Angeles offensive, John is notified by his army unit, Tech-Com, that Skynet will attack him from two fronts, past and future, and will ultimately change warfare forever. In response, he sets up 2 units; the Colorado Unit which is tasked with destroying the heart of Skynet, and a second unit lead by himself and Kyle Reese which is tasked with destroying Skynet’s final weapon, a time machine.
Before the resistance can reach the time machine, Skynet sends a T-800 back to 1984 to kill Sarah Connor, John’s mother. John agrees to send Kyle Reese back to protect Sarah. As Kyle is floating in the machine’s magnetic field, he experiences new memories of his 12-year-old self with a young Sarah Connor, and a warning that “Genisys is Skynet” and it will come online in 2017.
The T-800 arrives in 1984 Los Angeles, but is quickly dispatched by Sarah Connor and the Guardian, a reprogrammed T-800 with aged human tissue. Shortly after, Kyle is ambushed by a T-1000 and is rescued by Sarah and the Guardian. Kyle is informed that the original past has changed, Sarah already knows about Skynet, Judgment Day, and Reese’s role as her protector and John Connor’s father (although Kyle doesn’t know he is John’s father). They lure the T-1000 to a sewer, where Sarah disintegrates it with acid.
Sarah explains to Kyle that the Guardian was sent back in time to 1973 in order to protect Sarah the T-1000. The arrival of both terminators caused a fractured timeline that differs from Kyle’s. After saving and protecting Sarah, the Guardian raised and trained her to face her destiny, which she adamantly tries to reject. They have built a time machine to time travel forward to 1997 and stop the creation of Skynet. However, Kyle, convinced by his new memories, persuades the Guardian and Sarah that they should travel to 2017. Kyle and Sarah time-travel, while the Guardian spends the next 30 years preparing for their arrival.
Kyle and Sarah materialize in the middle of a highway and are captured by the police, followed by the Guardian. While they are being treated for injuries, Sarah is introduced to Genisys, a soon-to-be-unveiled operating system that will be used on every computer, mobile device, and military application. They are unexpectedly rescued by John Connor. As John leads the pair to the parking garage, the Guardian catches up with them and shoots John, revealing him to be a nanomachine replication of John, the T-3000. After a brief battle, the T-3000 is temporarily incapacitated with an MRI machine.
The Guardian takes Kyle and Sarah to a safehouse near Golden Gate bridge where they prepare to destroy the Genisys building before Genisys is online. The T-3000 tracks them down, and the battle reaches the Golden Gate bridge, where Kyle and Sarah are once again captured, this time with the Guardian. They are freed by Detective O’Brien, who was saved by Kyle in 1984, and is convinced that they are time-travellers with good intentions. The three hijack a helicopter and head to the Genisys building, pursued by the T-3000.
The T-3000 enters the Genisys building first, kills all the security guards and moves the count down forward from 13 hours to 15 minutes. The Guardian, Kyle and Sarah plant bombs at key points in the facility, all while holding off the T-3000. After a lengthy battle, the Guardian successfully traps the T-3000 in the magnetic field of the prototype time machine. After Kyle and Sarah navigate to a bunker beneath the facility, the time machine explodes, setting off the bombs and leveling the building, preventing Genisys from coming online.
With their mission over, the Guardian, Kyle and Sarah travel to young Kyle’s home, where he repeats the warning of Genisys to young Kyle. Sarah is relieved to know that she is now free to choose what to do with her future, and chooses to be with Kyle. The three then drive away. In the epilogue, the Genisys system core hidden in a subterranean chamber survives the explosion and the holographic projection of the now self-aware Skynet looks on.