This might represent a spoiler of some sort.
Ready? Here goes:
This guy, Marcus, a murderer with a heart of gold, is sentenced to death.
Helena Bonham Carter has cancer, so she turns up to buy his body off him for a kiss. Srsly.
15 years later, he wakes up in Mad Max's backyard.
Kyle Reese is busy being an adolescent, protecting the feral kid from Thunderdome, so they team up.
A lot of running around and blowing things up happens, sometimes involving a woman who has no purpose except to be rescued from stuff. Really Big Terminators are involved, and we get to see the first ones that look like motorbikes. Given that a lot of them can fly, these ones seem a bit of a downgrade and they can't do much apart from move forward or back, and I think one of them fires a gun once.
Marcus beats all the terminators but Kyle Reese and Star (did I mention the kid was named after the girl from Lost Boys?) are put in the Prisoner Transport Flying Thing.
Marcus is exposed as a terminator when brought to the rebel base. People don't like him. He escapes with the woman, but allows his hand to get shot for no particular reason.
Michael Ironside is in a submarine. John drops out of a plane into the sea and hopes that the submarine is nearby. It is. Lucky guy. Michael Ironside has discovered that you can turn terminators off by playing a short wave thing at them, which is sometimes a radio signal and sometimes audible, presumably depending on which person was in charge of editing the film at the time. There is no explanation for this signal, apart from "they're machines. You can turn machines off!" and several close-up shots of oscilloscopes. This makes no sense.
Marcus rescues John from sea serpent terminators that are in his fish pond. The pond he landed his helicopter on. Srsly.
Marcus goes to skynet. He gets there in 0 seconds, despite it being a couple of days away by Flying Thing.
John leaves to go to skynet. It takes him much longer, because he's riding a motorbike. Actually, he's riding a wheeled terminator, because capturing one of those and overriding its controls is much easier than using one of the vehicles abandonned at the side of the road, even though he presumably has to control it by pressing the touch screen of his iphone repeatedly. Because Terminators Don't Have Handlebars. He also has to sit on the terminator's face. Because Terminators Don't Have Fucking Sadles. Srsly.
Marcus is a terminator but still runs around skynet hiding behind things a lot. Terminators frequently ask him what the fuck is wrong, have a nice day.
Skynet rebuilds Marcus, he wakes up and wanders around until he logs on to the Big Control Room With Seats And Shit Clearly Designed For Humans. Helena Bonham Carter, re-written as a Flash applet, monologues for a bit to reveal the back story to Marcus, who gets mad and tries to break skynet by throwing a chair through the monitor.
We discover that the fuel cells that power terminators are nuclear, and can be blown up with a bit of fuse wire, which is handy because John has a roll in his pocket.
An Arnie fights John, and gets the hot and cold treatment from T2 all in one scene. For some reason it ends up scarring his cheek to match Marcus. I have no idea why.
The Arnie scans Marcus and determines his heart is vulnerable, so he punches it and Marcus immediately dies. Including all his robot bits. John revives him later with some jump leads. John gets a metal stake through the heart in the fight but it only slows him down, due to him being Hard.
Marcus carries John out of the nuclear blast zone and everyone likes him now.
In the tent, John gives some manly nods then starts to die. Marcus donates his heart to John. The field medic performs a heart transplant with a damaged heart that's been exposed to the elements for hours in a tent with the kind of hygeine in place that leaves everyone wearing the same clothes and covered in the same shit. Immediately afterrwards, John is fine apart from a bit of a cough, and is chatting with people in the helicopter taking him into the sunset. Aparently their technology was up to the task, but a heart from any of the dying or dead at the roadside was a no-no, basically because having Marcus around might cause continuity problems.
There's a whole subtle subtext about not killing people or being as bad as machines. If you stick your fingers in your ears every time John talks you might -maybe - miss it. I'm not going to talk about it.