In 1998, three years after averting the threat of Skynet, Sarah and John Connor are enjoying life by a beach in Livingston, Guatemala, when they are suddenly attacked by a T-800 Terminator. One of several sent back through time by Skynet prior to its erasure, the Terminator kills John before leaving. In 2020, an advanced Terminator, the Rev-9, is sent back in time to Mexico City, to murder Daniella "Dani" Ramos, who will become a threat to the machines in the future. However, a cybernetically enhanced soldier, Grace, is sent from 2042 to protect her. The Rev-9, disguised as Dani's father, infiltrates the automobile assembly plant where Dani and her brother Diego work, but is thwarted by Grace, who escapes with the siblings. The Rev-9, using its ability to split into its cybernetic endoskeleton and shape-shifting liquid metal exterior, pursues them, killing Diego and cornering Grace and Dani. However, Sarah arrives and temporarily disables the Terminator with explosives. Dani, Grace, and Sarah retreat to a motel. Sarah reveals that she found them because, in the years since John's death, she has received encrypted messages detailing the locations of arriving Terminators, each ending with "For John", allowing her to destroy them to avert Skynet's return. Grace notes that Skynet and John do not exist in her future. Instead, humanity is threatened by an AI called Legion, originally developed for cyberwarfare. When Legion became a threat to humans, an attempt was made to neutralize it with nuclear weapons, resulting in a nuclear holocaust and the AI creating a global network of machines to terminate the human survivors. Grace traces Sarah's messages to Laredo, Texas. Barely evading the Rev-9 and the authorities while crossing the Mexico–United States border, they arrive at their source, where they discover the same T-800 that murdered John. Having fulfilled its mission and with Skynet no longer existing, the T-800 was left aimless. Through learning, however, it became self-aware. During that time, it learned from humanity and developed a conscience, taking the name "Carl" and adopting a human family. After learning how its actions affected Sarah and being able to detect the location of temporal displacements, Carl decided to forewarn her of them to give her purpose in an attempt to make amends. Carl offers to join them against the Rev-9, and they make preparations to destroy it, with Sarah begrudgingly agreeing to work together for Dani's sake. Anticipating the Rev-9's arrival, Carl bids its family farewell and tells them to escape. They seek out a military-grade electromagnetic pulse (EMP) from an acquaintance of Sarah's. The Rev-9 catches up with them, forcing them to steal a C-5 Galaxy to escape, though the EMP's are destroyed in the resulting shootout. During the flight, Grace reveals that Dani becomes the future founding commander of the Resistance, as well as her rescuer and adoptive mother. The Rev-9 boards their airplane via a KC-10 Extender and temporarily subdues Carl, forcing Grace, Sarah, and Dani to jump from the plane into a river near a hydro-power plant, with Carl and the Rev-9 following close behind. Bludgeoned, the group makes their stand inside the plant. In the ensuing battle, Carl and Grace force the Rev-9 into a spinning turbine, causing an explosion that critically damages the two Terminators and mortally wounds Grace. The severely damaged Rev-9 endoskeleton incapacitates Sarah, forcing Dani to confront it herself. A dying Grace tells Dani to use her power source to destroy the Rev-9. Dani tries to fight it, but is quickly overpowered. Carl reactivates itself and restrains the Rev-9, allowing Dani to stab it with Grace's power source. Carl drags itself and the Rev-9 over a ledge, right before the power core explodes, destroying them both. Sometime later, Dani and Sarah watch a ten-year-old Grace at a playground with her family, the former determined to avert Grace's death and Legion's rise. Sarah then tells Dani she needs to get ready, with Grace watching as the pair drives off.