The movie starts out in 2003 with Marcus Wright (Sam Worthington) in a jail cell. A doctor named Serena (Helena Bonham Carter) comes in to speak with him, offering him one more chance to donate his body to science so that other people could benefit from his healthy tissue and organs, and he could have a "second chance at life." He becomes angry, yelling that he doesn't deserve a second chance. We learn that he was about to be executed as a result of killing his brother and two police officers. After a bit of back and forth he retorts, "I can't cure your cancer." Serena states that she isn't trying to help herself, she is trying to help the world. He eventually relents and offers to sell his body to her for a kiss, after which he states, "So that's what death tastes like." He is then executed.
We then jump to the year 2018 where the resistance is attacking a Skynet base. John Connor (Christian Bale) is among the ground troops, and we discover their mission is to infiltrate the underground base and download some information from their computers. After taking out a few terminators they make it inside, but it seems too quiet to them. While trying to find the computer system, they find hundreds of human prisoners, which they find odd since “Skynet doesn’t take prisoners”, and relay the information back to command. Upon location of the computer, and the subsequent access of the information they are trying to recover, John notices a file about a new series of terminator that was made of human tissue and could regenerate, the T-800. General Ashdown (Michael Ironside) is demanding to speak to the troops topside, but they don't respond, so John is sent back to find out what they're up to. Upon reaching the surface, however, he realizes things aren't right, as all the resistance forces are dead. There is a transport carrying a bunch of humans, so John takes off in a helicopter to pursue it. After taking off, there is an explosion in the complex and the shockwave causes the helicopter to go down. He is then attacked by a T-600, which he ultimately destroys by utilizing the machine gun turret attached to a helicopter. When John radios in for extraction he is asked how many survivors there are. There is only one, himself. However after he is picked up, we see someone climbing from the wreckage. It is Marcus Wright.
While being transported, John demands to be taken to command. When the pilot refuses, he tells him to lower the ramp and radio into Command to send divers for a lock-in. On the submarine, John demands to know what information they retrieved as all of the men sent in died except for him. Ashdown explains that commands are sent out to all terminators using the same communications signal, and that they have discovered a signal hidden within that could be used to shut down the machines. John demands to test it out, as he is the man for the job. The general agrees and they say they will have it ready in four days. When John asks why four days, he is told that they intercepted a kill list and everyone on it would be dead within a week and that he is number two. He asks who number one is and is told "A civilian. I don't recognize the name. Kyle Reese."
Next, Marcus is seen walking around an abandoned city. He calls out when he sees someone, but it turns out to be a T-600, which begins to attack him. A young man (which we find out is Kyle) shows up and tells Marcus to "Come with me if you want to live." The city is rigged with various traps to use against the terminators, which Kyle is forced to deploy since Marcus caught their attention. We meet a young girl named Star (Jadagrace) who seems to be a mute, who helps Kyle destroy the T-600 and then seemingly detects an incoming Hunter-Killer (referred to as H-K through the rest of the spoiler) and everyone is forced to lie flat and unmoving while it passes overhead. Marcus asks what it was and Kyle explains stating "Thanks to you, now they know we're here." We find out that Kyle and Star are the LA unit of the resistance. Marcus wants to go to San Francisco and Kyle tells him he'll need a car. He knows where some are, but none of them work. Marcus wants to go find the car right away, but he is stopped by Kyle who tells him the H-Ks have infrared and hunt better at night.
We cut to John returning to base and informing Barnes (Common) that his brother didn't make it. He provides them with the signal to shut down the terminators so that they can test it on something small. One of them suggests a hydrobot. We cut back to Marcus who fixes a radio in time to hear John making a broadcast while resistance members all over the country listen. He talks about vulnerability in the T-600 model. If it is stabbed in the back of its neck, its targeting system goes haywire and it is unable to shoot its intended target. He mentions his name and Kyle states, "We have to find this guy." Back at the base, John is talking with Kate (Bryce Dallas Howard) and tells her "this isn't the future my mother prepared me for." Evidently previous events have altered the timeline and as a result progress is being made on the T-800 line ten years earlier than was originally thought. Testing the signal on a captured hydrobot, they find that it is possible to shut them down, but the signal needs to be continuous or the machines will go right back to attacking again.
Back with Marcus, he is with Kyle at the location with the cars and is almost done fixing it. When he gets it working, the radio begins to play "Rooster" by Alice In Chains. Kyle asks what it is and Marcus replies "Something my brother used to listen to," before turning the radio off. He demands Star get out of the Jeep as he wants to leave without Star and Kyle, but they are found by a scout (it heard the music playing) so he takes off with them. To destroy it Marcus has Kyle drive while he throws a wrench at it.
We next see them at an abandoned 7-11. They are trying to get some gas, but the resistance there refuses until an old lady comes out. She provides them with food, after which one of the resistance turns on her, pointing a gun asking what gives her the right to give away their supplies. All of a sudden, a giant mechanical hand breaks through the roof and grabs her. It is a harvester and it begins grabbing all the people it can. Some try to flee in cars, but they are shot at until the cars explode, killing them. Marcus sees the transport full of people and comes up with a plan to open the valve on the fuel tanker and create a line leading to the harvester. He uses a tow truck to push the tanker right next to the harvester and tells John to shoot at the truck, but he fails to ignite anything. Star provides Marcus with a flare, which he throws, igniting the line of fuel and causing a massive explosion as they drive away. However a few seconds later, the harvester emerges undamaged, and deploys two moto-terminators to go after them. Naturally, they are equipped with the ability to recover from crashes and recalculate their paths when there is an obstruction. Kyle disposes of one by throwing a container of oil off the back and shooting it, which makes the road slick causing a collision. Approaching a bridge, which is blocked by an H-K. To destroy the other moto-terminator, Marcus has Kyle deploy the tow-hook, resulting in the moto-terminator becoming tangled and flung at the H-K, destroying both. At this point they think they're finally safe, but then Kyle and Star are grabbed by a Harvester and dropped into a transport.
Resistance pilots then show up and begin to shoot at the machines, while being pursued by H-Ks. Marcus is attempting to break open the transport to free the prisoners, when it takes off and he falls into a river, skipping across it like a stone. One resistance pilot gets shot down, and the other can't shake the H-Ks following her. Finally she escapes by ejecting shortly before the plane explodes. When we next see Marcus he is approaching the ejected pilot, Blair Williams, (Moon Bloodgood) who he helps free from her parachute. She thanks him and he asks where the transport was going, as he wants to go after it to rescue the prisoners. She tells him to just forget about it as they're most likely already dead, but he insists. She tells him that they can go back to her base, and John Connor might be able to help, but it is a two-day walk back. Stopping for the night, Blair sends Marcus to find something he can burn, while she takes care of her injuries. Suddenly she is ambushed by some scavengers who she fights off at first (a 3 on 1 fight no less), until they get the upper hand, at which point Marcus shows up and begins taking care of them with ease. Blair seems to be taking a liking to him and tells him she is cold. When he does nothing, she gets closer and begins hugging his body for warmth. She listens to his heart and tells him he has a strong heart, and that she loves the sound. She thanks him for helping her, as she doesn't meet many good guys anymore. Marcus replies that he isn't a good guy, and then asks her if she believes in second chances, to which she replies yes.
We cut to a quick scene at some sort of prison camp where the transports are being unloaded. One panics and tries to escape, but is shot to death by a T-600 guard. We next cut to Barnes and John at the Skynet valley of death. Barnes comments that they've never been this deep into Skynet territory before. John tells them that they had to go there to test their transmitter on something bigger. He triggers a remote explosion and after a little while an H-K flies in to survey the situation. John has Barnes turn the signal up to full power and after a bit it shuts down. They realize that they might actually win the war thanks to the transmitter. In a conversation with General Ashdown (via radio), John gets upset that he is planning to use the transmitters to get close enough to the Skynet facility to bomb it. Knowing there are humans in there he says that if they bomb it, killing all of them, they are no better than the machines. After a heated argument, Ashdown finally tells him he that he is relieved of his command. Barnes states to John "I didn't hear that last part," and a few others chime in that they didn't either. John begins transmitting to all the resistance forces across the country again, telling them that they are soon going to be sent to bomb the base. He asks them to ignore the order and give him a chance to try and save the prisoners.
Finally, Blair and Marcus are close to the base. They approach a minefield, and Blair mentions that they're magnetic, so as long as they walk due west, they shouldn't have any problems. While walking over the mines, we start to see them rattle a bit, and suddenly one sticks to Marcus's leg and explodes. While he is being operated on, Kate asks if he has a prosthetic limb, and suddenly they discover that he is a terminator! Barnes knocks him out, and we next see him hanging from chains. Kate mentions that he may have been human at one point. He is a new hybrid machine with human organs, human tissue, and a hybrid brain made up of one human cortex and one machine cortex. John confronts him and tells him that he will not succeed in destroying them. Marcus replies that he is just trying to save Kyle and Star. John is taken back a bit at the mention of Kyle. Blair comes to his defense, saying he isn't one of them, and that he saved her; but all the others disregard it, stating that is what Skynet wants them to believe. Barnes is shooting at Marcus when Blair comes in and tells him John wants to see him. She takes the gun and shoots Marcus and Barnes leaves. She then lowers the chains and jumps down after Marcus and begins to free him. John is listening to the tapes recorded by Sarah Connor and talking to Kate when there is a knock at the door. It's Barnes who says, "You wanted to see me?" John immediately knows something is wrong and runs back to where Marcus was being held. They attempt to stop Blair and Marcus from leaving, but are unsuccessful. Barnes prepares to shoot a rocket launcher towards the tunnel they're escaping through and he is asked "What about Blair?" to which he responds that she made her choice. There are many problems ahead for Blair and Marcus trying to flee as all the guards are on the lookout. Blair detonates a line of mines, clearing a path for them, but they are soon blocked yet again. Marcus draws fire from troops in order for Blair to shoot out their lights making it harder to track them. Blair acts as a decoy, lying on the ground, making the resistance troops think she is Marcus. When they realize it's actually Blair, Marcus is already well on his way out. At one point during all of this, Blair is shot in the leg. John is in a helicopter, surveying the river to look for Marcus. Hydrobots begin to leap out of the water and attack the troops onboard, causing it to lose control and go down. All alone after the rest of the crew is killed, John is fighting off the machines, when he runs out of bullets. Just as one is about to reach John, Marcus leaps out of the water and snaps it in half. John is safe on land, and the hydrobots won't attack Marcus, as they know he is another terminator. John still doesn't quite trust Marcus, but agrees to let him go back to Skynet to help him free Kyle, providing him with a means of contacting him once he accomplishes his task. He lets him go, and the others arrive asking where he is. John simply states that he's "gone."
We next see John preparing to head towards the Skynet facility in San Francisco. Kate asks him what she should tell the rest of his men if they ask. He says, "I'll be back." John turns on a radio and starts setting up a trap to catch a moto-terminator. When he successfully gets it, he connects to it with some type of computer (seen at the beginning when infiltrating the Skynet facility) and reprograms it so that he can ride it to Skynet Central. We cut to General Ashdown giving the order to launch the assault on Skynet, but all of the troops refuse. They won't act until John gives the order. Marcus arrives and seems disappointed when T-600s see him, but simply let him pass. He is beginning to realize that he is much less human than he thought himself to be. When Marcus reaches the main computer, he syncs with it and we see flashes of his past as he is absorbing the information. The main point of interest is that he was created by Skynet using Serena's research. He begins lowering the defenses, and lets John know that it's all clear. We find out through the main computer (displaying the image of Serena) that Skynet created Marcus as an infiltration unit. With his help, they were able to track down Kyle Reese as well as John, who was currently on his way into the facility. Unknowingly, Marcus executed his program exactly as he was intended to, dooming John, Kyle, and all of humanity. We learn that the shutdown signal was also a trap. Skynet purposely let the resistance find it so that they could draw them into a false sense of security. We cut back to the command submarine and radar shows a large bogey incoming. They had been broadcasting the signal to shut down the terminators all along, and Skynet used it to track them down. An H-K flies in and fires at the sub, killing everyone inside. Marcus learns that he didn't want to go to San Francisco to track down the person who made him; he was actually programmed to go there so that Skynet could destroy John.
Meanwhile, John has reached his point of entry, and hacks into the system to open a door, narrowly missing being caught by a patrolling T-600. He begins freeing the prisoners and looking for Kyle, but it seems Kyle is still trapped in his cell. We cut back to the computer as Serena asking Marcus if he knows what he is. He replies that he does, and rips the Skynet control chip from the back of his head before destroying the computer, and heading out to help John. We see that a T-600 is outside of Kyle's cell and when it opens he makes a break for it. He stabs it in the vulnerable point in the neck that John had talked about earlier, allowing him to get away. John is still looking for Kyle when he opens a cell and out walks the T-800 model 101 (aka Arnold). It destroys the T-600, taking its gun in order to fire at John. Running away, John finally finds Kyle (and Star), but they don't have much time as "Arnold" is after them. Firing multiple grenades at it does nothing, and it continues to pursue them. They find themselves in the factory where the terminators are being produced. They find the terminators' fuel cells and John mentions that they are nuclear and enough to level the whole facility. They carefully rig them with explosives and get on an elevator back to the surface. At the last second John gets off and sends the elevator up. Kyle yells, "No! I won't leave you!" and John replies, "You didn't." The T-800 uses its speech synthesis to sound like Kyle and trick John into revealing his location and an epic battle ensues, with John getting thrown all around the facility. He notices the tank of molten steel and shoots a grenade at it, causing it to pour over the T-800. Unfortunately, this model 101 seems to be stronger than the one John encountered in his past, as it emerges with no damage other than its skin missing. John then freezes the terminator in its tracks by shooting a duct, which sprays freezing air all over the molten steel it is covered in. This too doesn't stop it, as it begins to break through and scratches John's face. Finally, Marcus shows up and begins fighting with the T-800. It seems despite his strength, he too may not be a match for the model 101. It scans Marcus and notices his human heart, which it sees as vulnerability. One quick punch to the chest later and Marcus is dead. John scrambles to rip power cables out of one of the machines producing the terminators and electrocutes Marcus, trying to shock him back to life. He finally succeeds, but it is a bit too late as he is stabbed through the chest just as Marcus regains life. Marcus then breaks part of the metal rod off, shoves it through the T-800's neck and breaks its head off. He returns John to the surface and they take off in a helicopter. Star hands him the detonator (it had fallen at some point during their run out of the building, and she picked it up) and they destroy the facility.
John is lying on a gurney, still wounded, surrounded by Kyle, Star, Barnes, Blair, Marcus, and Kate. Kate turns to talk to Barnes and tells him that his heart can't take it. Marcus says, "Then take mine." Everyone is shocked, and Blair is especially affected by the decision. He says that everyone deserves a second chance, and this is his. Star grabs his hand and we then see Kate prepping him for the surgery. He goes under anesthesia and the movie ends shortly after with narration from John stating that the resistance had won the battle, but the war against the machines was just beginning. Skynet Central was destroyed, and it was a major victory, but there were still many more Skynet bases across the country.
*CUT TO THE CHASE*
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Marcus Wright was executed and given a "second chance at life" by donating his body to science. However, his second chance turned out to be becoming a terminator. He was created and programmed as an infiltration unit, so that Skynet could find and destroy John Connor and Kyle Reese. Being a hybrid human/machine, he still 'felt' human and revolted against Skynet, ripping out the Skynet control chip from the back of his head. A new terminator model (T-800 aka Arnold) was fighting with John, and Marcus intervened. Seeing his human heart as a vulnerability, the T-800 killed Marcus with a punch to the chest. John electrocuted him, jump-starting his heart, but the T-800 stabbed John through the chest with a metal rod. Marcus broke part of the rod and used it to break the T-800's head off, and took John and Kyle to the surface. Flying away in a helicopter, John triggered an explosion that leveled the Skynet facility. Back at base, John was lying on a gurnee, about to die as his heart was too weak. Marcus volunteered to be terminated and give John his heart. He called it his "second chance."
The movie ends with narration from John stating that the facility was destroyed and the resistance had won the battle, but the war against the machines was just beginning as Skynet had many more bases.
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