In 2003, a death row inmate named Marcus Wright at the Longview Correctional Facility in Texas meets with representative Dr. Serena Kogan from Cyberdyne Systems Genetics Division to donate his organs and tissue for what she believes will be a noble cause (the future of humanity). He agrees to do so in exchange for a kiss. Afterward, he is put to death by lethal injection. The last image he sees is Dr. Kogen standing over him.
In 2018, John Connor participates in a major Resistance assault a Skynet VLA network in order to obtain classified data vital to the machines and rescue the prisoners there. Connor would leave after having infiltrated the base to try to rescue several prisoners being taken by a Skynet transport. However, he discovers the blueprints for a new enemy to the Resistance — the Series 800 Terminator, which should be produced in the year 2026. This would be his salvation as the base explodes moments after he leaves due to a Skynet booby trap.
John is be picked up a short time afterward and requests to be taken to Resistance Headquarters. He is denied an audience with the Resistance leaders, but jumps out of the chopper and swims down to the Resistance Headquarters — which is located on a submarine. After his arrival, he is denied entry, but forces his way in. While there, he meets with General Ashdown and tells him what he'd found and that Skynet was more advanced than it should be. The leaders are unhappy with him but tell him they have discovered a hidden frequency that could potentially shut down Skynet for good. John volunteers to test the signal on progressively more advanced machines. The Resistance has also intercepted a transmission from Skynet: a kill list warning that all the Resistance leaders will be dead in four days. John himself is second on the list, with Kyle Reese as their Number One target.
In the meantime, Marcus wakes up 15 years after his execution, dazed and confused at his surroundings. After taking clothes from a dead Resistance fighter, Marcus makes his way through the desert and into Los Angeles, only to discover the city is in the ruins, and the only human survivors there are a young Kyle Reese and his young, silent companion, Star. After rescuing Marcus from the onslaught of a T-600, Kyle explains how the Judgment Day happened as he knows it and that he is working to become a full-fledged Resistance member. So far, though, Kyle and Star haven't had much luck finding others to join their branch. Marcus, determined to help, fixes an old shortwave radio, which, by chance picks up a broadcast from John Connor, whose daily broadcasts are secretly focused on finding Kyle, his future father, as well as giving hope and providing advice on how to fight Skynet's forces to the Resistance and human survivors around the world.
As Marcus and Kyle leave the city behind to meet John Connor, they encounter a 7-11, housing humans who, instead of actively sabotaging Skynet activity, "keep their heads down" to avoid attracting the attention of the machines. They deny gasoline at first, but an old woman, apparently the leader, asks them to stay, offering something to eat. During this, the 7-11 suddenly comes under attack by a Harvester, taking several prisoners. Though the three try to escape, Kyle and Star are still captured eventually. Marcus attempts to follow, but is knocked off by the Harvester on the Transport.
Meanwhile, Connor and his lieutenants, including his right-hand man, Barnes, and his second-in-command and wife, Kate Connor have never heard of Skynet taking human prisoners, and he is determined to discover the reason. They pick up machine activity in the Los Angeles Basin and dispatch two A-10 Warthogs to intercept the Hunter-Killers trying to escape the area. However, two attackers were shot down by the machines. Blair Williams, one of the pilots, successfully ejected and later rescued by Marcus, while the interception fails and Connor is ordered to follow through with a Skynet infiltration attack. John Connor and Marcus Wright Gideon9595Added by Gideon9595
Blair and Marcus bond over a two-night trip back to her base, in which, after detonating a magnetized mine, Marcus is revealed to be of a machine. His brain and heart are the only organs left, thought to be the prototype for infiltration Terminators like the T-800.
Blair, unable to accept that Marcus is part of Skynet, releases him after which the Resistance chases them down, with Blair being captured to allow Marcus to escape. Connor chases after him in a Blackhawk, but suddenly gets brought down into a river by Hydrobots. Before Connor is killed by a swarm of Hydrobots, Marcus saves him. Connor makes a deal, and with his permission, allows him to enter the heart of Skynet Central, in exchange to get Connor into the base, so he can free Kyle. Marcus makes his way to Skynet and shuts down the turrets guarding a huge wall, and Connor is able to enter the facility.
While Marcus is deactivating the security, his machine circuitry syncs with the Skynet mainframe and causes him to black out. When he comes to, he is completely repaired, and Skynet, taking on the appearance of Dr. Serena Kogan, reveals his true purpose. Skynet tells him that he is only a Trojan horse of sorts, whose intent was to identify Connor and Reese and lure them into Skynet, where they could be killed. With his mission fulfilled, Skynet expects him to accept he is now a machine and will continue fighting alongside Skynet. Choosing to be a human, Marcus rips out his main chip and helps Connor locate Kyle. Skynet warns that fighting is useless and that he cannot save John and Kyle. Marcus snarls "Watch me!" before smashing Skynet's monitor, escaping the chamber.
Meanwhile, General Ashdown and the other Resistance leaders discover that the signal they had implemented to destroy Skynet was really a location beacon. An HK-Aerial tracks the sub in which they are in and drops a depth charge, destroying it.
While John is searching for Kyle Reese at Skynet Central, he discovers a T-RIP with the mission to kill John Connor and Kyle Reese above all — and chases them throughout the Terminator Factory beneath the complex. Kyle is captured by a T-600 but escapes by jamming a rod into the Terminator's motor cortex before it can kill him. As the T-RIP enters the room in pursuit of John and Kyle, it easily destroys the damaged T-600 blocking its path. John fires several "Hellhound" 40mm grenades at the T-RIP, slowing its pursuit and burning off its flesh covering. They escape by blowing a hole in a nearby wall.
The T-RIP catches up with John and a fight ensues but their battle is interrupted when Marcus intervenes, allowing John to get Kyle to safety. The T-RIP battles Marcus in single combat with neither gaining a distinct advantage. However, Marcus is defeated by the T-RIP, who discovers that his human heart is vulnerable and delivers a devastating blow to his chest. Marcus collapses and the T-RIP resumes its chase of Connor. John fires a grenade at a vat of molten metal, which pours out over the T-RIP. Because the molten metal can't stop the machine, John then fires at a coolant line with his pistol, which freezes the T-RIP solid.
John then tries to revive Marcus with a few solid thumps to his chest. When this fails, he rips away electrical conduits and shocks Marcus for several time to bring him back to life. However, this gives the T-RIP time to breaks free and, while he is distracted, impales John through the chest with a steel bar. Marcus, breaking the steel bar and ramming it though the neck of the T-RIP and decapitating it with ease. He carries John to a waiting helicopter, where Kate and Barnes having assembled an evac force for the human survivors. Then Star offers Connor the detonation controller.
Once they are airlifted from the blast site, Connor detonates the detonating cord that he had previously wrapped around several nuclear fuel cells in the Terminator Factory and destroys the Skynet Central before falling unconscious, due to the mortal wound to his heart from the T-RIP. In a base camp, Marcus resolves to give up his heart to save John. He explains to Blair and Kyle that everyone deserves a second chance on life, including Connor, and willingly undergoes a heart transplant.
When Connor wakes up, he is on a helicopter flying towards the horizon along with a fleet of Bell UH-1 "Huey" Helicopters.
The film ends with John quoting his mother, saying, "There is no fate but what we make". He warns the Resistance and other survivors across the world that while they have won this battle, the war is by no means over: Skynet's forces still remain strong, and humanity has a long way to go before it achieves ultimate victory over the machines.
- Christian Bale as John Connor. Director McG deemed Bale "the most credible action star in the world" during development. McG wanted Bale for Marcus, but the actor—even though he "can't really remember why"—wanted to play John, and that led to the character's role getting expanded in rewrites of the script. Bale was the first person to be cast and signed on for the role in November 2007. McG talked extensively with Bale in the UK about the role while the latter was filming The Dark Knight, and they both agreed to proceed. Although a fan of the Terminator series, he was at first uninterested until McG convinced him the story would be character-based and not rely on special effects. They kept working on the story every day, along with Worthington. McG said Bale broke his hand punching a Terminator prop during filming. Bale also spent six to eight hours each day with McG in the editing room to advise the finished product.
- Sam Worthington as Marcus Wright, a Skynet human-terminator hybrid experiment. Worthington compared Marcus to Dorothy Gale and Alice due to being "this person waking up in another world [who then] tries to find himself". Terminator creator James Cameron personally recommended Worthington (due to him starring in Avatar) to McG. Russell Crowe also recommended him to McG. The director decided Worthington looked tougher than the "great many of today's [waify] young male actors". Worthington recalled Cameron told him "the Terminator to make is the one with the war". Worthington tore his intercostal muscles during the first weeks of filming, but nevertheless insisted on performing his own stunts. McG once expressed interest in casting Christian Bale, Daniel Day-Lewis or Josh Brolin in the part. Brolin did talk to Bale and read a draft of the screenplay, which he found "interesting and dark; ultimately, though, I didn't think it felt right".
- Anton Yelchin as Kyle Reese
- Moon Bloodgold as Blair Williams
- Bryce Dallas Howard as Kate Connor, John's wife
- Common as Barnes, John's right-hand man
- Helena Boham Carter as Dr. Serena Kogan/Skynet
- Michael Ironside as General Ashdown, leader of the Resistence
- Roland Kickinger as The Terminator / T-800 (Model 101)
- Jadagrace Berry as Star, a nine-year old girl cared after by Kyle Reese
- Terry Crews as Captain Jericho, Barnes' brother
- Linda Hamilton as Sarah Connor, John's mother. Sarah's voice is heard from tapes she recorded before her death prior to the film's events.
- Common as Barnes