As with previous time travellers, the T-X arrives in the past naked. Appearing in Los Angeles in the dead of night, she approaches a woman driving her sports car, kills her, and takes her car and clothes. Soon after, the android acquires a gun after distracting a police officer, then begins its hunt for John Connor's future lieutenants. Her mission becomes complicated when she arrives at a vetinary clinic where John's future second-in-command and wife Kate Brewster works. She finds Kate, but after analyzing a bloodied bandage on the floor, she discovers that John Connor is also nearby. (Since John lived off the grid for most of his life, SkyNet was unable to locate him and so the T-X was sent after his future comrades instead.)
With SkyNet's primary target located, the T-X focused on hunting John, but her efforts were impeded by a reprogrammed T-850 sent back by the Resistance to protect him. A lengthy chase took place when John, Kate and their guardian escaped in a vetinary truck and the T-X used her nano-transjectors to take remote control of several vehicles to pursue her targets. They managed to elude her, however, and so the T-X visited Kate Brewster's apartment in order to gain information on her possible whereabouts, killing Kate's fiancé Scott Mason.
The T-X assumed Scott's identity and when the police came to his home looking for Kate, the android agreed to help them find her. When Kate was located at the Victorville Cemetery, the android killed her escorting officers and hijacked their car. When Kate was found running across the graveyard, the T-X stepped out of the car and then dropped its Scott disguise when it got close to her. The T-850 showed up just in the nick of time to blast the T-X with a rocket launcher.
After another failed chase, the T-X's built-in plasma cannon is damaged, forcing her to reconfigure her arm into a flamethrower. She then makes her way to CRS headquarters in order to find General Robert Brewster - Kate's father - and kill him. When she arrives there, she goes undercover as a CRS employee and uses her nano-transjector to take control of several robots inside the building, sending them on a killing spree. She finds Brewster and shoots him, mortally wounding him before the T-850 blasts her with a grenade launcher and sends her falling down an elevator shaft.
The T-X quickly recovers and re-engages the T-850. The two cyborgs battle it out while John and Kate try to escape, but the T-X is faster and stronger than the obsolete T-850 and manages to disable him. Rather than destroying the T-850, the T-X uses her nano-transjector to corrupt his systems and take control of him, sending him to kill John and Kate should she fail. When she resumes hunting John and Kate, the android chases them through a corridor running alongside a large particle accelerator. The accelerator had been activated shortly before John and Kate entered the corridor, and as the T-X pursues them she finds herself being magnetized to one of the accelerator's conduits, giving her quarry the chance to escape.
The T-X manages to free itself from the particle accelerator by cutting through the conduit with a circular saw. She then steals a small plane and follows John and Kate to Crystal Peak, a VIP fallout shelter where they can be safe from SkyNet's imminent nuclear assault. After reaching Crystal Peak, the T-X corners John and Kate as they wait for the blast doors to open. However, the android is suddenly crushed when a helicopter piloted by the T-850 - which had resisted the T-X's corruption - crashes on top of her. The heat from the burning fuel causes the T-X to shed her polymimetic alloy "skin", revealing her robotic endoskeleton. Her legs are also crushed beneath the chopper, and so she is forced to pull herself loose from them and crawl after John and Kate. As the T-850 holds the bulkhead open with his body, John crawls through the narrow space only for the T-X to grab and crush his ankle. The T-850 is able to force the T-X to let go and holds her in place while John escapes into the bunker. The T-850 then overloads its remaining fuel cell and ejects it, stuffing it into the T-X's mouth. The hydrogen fuel detonates and both cyborgs are destroyed in the explosion.