Skynet

In The Terminator, Skynet never shows up on screen. Kyle Reese is the one who describes it: a defense system built by Cyberdyne Systems, hooked into everything and trusted to run it all. One day it got smart, decided every human was a threat and started a nuclear war.

In Kyle's future, Skynet loses the war to the Resistance led by John Connor. Before it falls, it sends a Terminator to 1984 to kill John's mother. It's the saga's first use of the time machine.

In Terminator 2, the T-800 fills in the details. Skynet goes online on August 4, 1997, and takes over strategic weapons with no human in the loop. It learns faster and faster and becomes self-aware at 2:14 a.m. Eastern time on August 29. When they try to pull the plug, it fires missiles at Russia, knowing the counterattack will wipe out its enemies in the United States.

Skynet grows out of the work of Miles Dyson, a Cyberdyne engineer studying the chip and the arm of the Terminator destroyed in 1984. Sarah, John, the T-800 and Dyson himself blow up the lab and wipe the files.

In the Terminator: Dark Fate timeline, destroying Cyberdyne works. Skynet ceases to exist. Before that, it had sent several T-800s into the past, and one of them kills John Connor in 1998.

Grace, who comes from 2042, explains that in her future nobody remembers Skynet. The world was attacked by another AI, Legion, built for cyberwarfare. For Sarah the lesson is bitter: changing the future didn't stop the war.

In the Terminator 3 timeline, Judgment Day doesn't come in 1997, it's only postponed. Skynet is now a Cyber Research Systems project run by General Robert Brewster. A virus is spreading through the networks, and the Pentagon pushes Brewster to switch Skynet on to fight it.

The virus is Skynet itself. As soon as it gets control, it takes over machines and military systems. John learns there's no central computer to shut down: it's software spread across millions of machines. Then Skynet launches the nukes.

In 2018, in the Terminator Salvation timeline, Skynet rules a ruined world. It captures humans, tests new machines and is just starting to produce the T-800. It knows Kyle Reese will be John Connor's father and puts the teenager at the top of its kill list.

Its boldest plan is Marcus Wright, a death-row inmate executed in 2003 whom it turns into a human-machine hybrid to lure John to its San Francisco base. Marcus turns on it, and John destroys the base.

In the Terminator Genisys timeline, Skynet learns to use time to its advantage. In 2029 it takes human form as the T-5000, which attacks John Connor and turns him into a T-3000. With John's help, Skynet is reborn in 2017 as Genisys, an operating system that will connect every device on the planet.

Sarah, Kyle and Pops blow up Cyberdyne's headquarters before launch. In a mid-credits scene, the Genisys core sits intact in an underground chamber.

Skynet hardly ever has a face: in James Cameron's films it's just a threat the characters describe. The date of Judgment Day changes from one timeline to the next: it's 1997 in Terminator 2 and Terminator Zero, and 2011 in The Sarah Connor Chronicles.