If you ever wanted to know the plot of starcraft I & II, you're in luck because I just decided to see if I can recite it from memory
Prehistory
At some point in a not too distant but counterfactual future where there is space travel but no AI, humanity exiles a bunch of criminals out to space as a punishment. They land on a planet cluster somewhere. They establish a civilization ruled by the confederacy, something like a big evil government.
The "Protoss" are an alien species with advanced tech (which does not translate into vastly superior military strength because then the story wouldn't work); they sometimes attack humans but there's no all-out war. No-one knows where they come from or what they want.
Campaign I
You're a low ranking officer in the confederacy in charge of some unimportant exploratory mission. You team up with the local Marshal called Raynor. A couple of planets are attacked by a different alien race that is entirely biological and hostile, later called Zerg. The confederacy, evil as it is, quickly abandons lots of their colonies that don't have military importance. You decide to save some unimportant colony by destroying a zerg thing. The confederacy is pissed because they wanted to study it. You decide they suck and team up with the leader of the largest rebel group called Mengsk. Together you save a bunch more people from the Zerg. Some woman named Kerrigan works for Mengsk and Raynor falls in love with her.
Inevitably some high ranking officer of the confederacy called Duke gets attacked by the Zerg and Mengsk orders you to save him. Duke subsequently joins your team. You also find out that the confederacy has been studying Zerg for a while before the attacks and controls artifacts that lure them. You steal one and place it into the center of the confederacy (also at orders from Mengsk) which of course gets overrun by Zerg. That along with a bunch of colonies joining the rebellion and the help of Duke who knows the defenses officer is enough to win the rebellion. You and Raynor get increasingly disturbed by how little regard Mengsk shows for human life. Then Kerrigan leads some mission, which ends up with her position being overrun by Zerg. Mengsk decides not to evacuate her for personal reasons (she actually killed his family, but that was when she was essentially a puppet working for the Confederacy, drugged and unable to make autonomous decisions; she was loyal to him because he freed her). Raynor and you are mad and part ways with him. Duke has no such problems and remains loyal to Mengsk. Mengsk declares himself The One True Emperor of the human race and Duke becomes something like the second in command.
Campaign II
The zerg race is controlled by a giant organism called the Overmind and a few lesser Organisms called Cerebrates, who are like giant brains that can't themselves move or fight. The cerebrates can think but are incapable of disobeying commands from the Overmind. All of the remaining zerg are mindless animals. You're a cerebrate. You oversee some cocon with something "very powerful" inside. It turns out it's Kerrigan who's been caught and is now being infested and turned into a zerg. (Their entire thing is to steal DNA from other species and mutate it.) Infested Kerrigan can think-but-not-disobey like the Cerebrates.
Cerebrates are traditionally supposed to be immortal (if they're killed, the Overmind revives them) but some subspecies of Protoss called Dark Templars can kill them for real. They kill one of the important Cerebrates. Kerrigan is happy about this because she didn't like him. The overmind makes brief contact with the mind of one of the Dark Templars and reads the location of the Protoss home world from his mind. The Zerg attack the home world and it devolves into war and chaos.
Campaign III
You're a high ranking protoss leader on their home world (currently at war). The protoss are ruled by the Conclave, who thinks the Dark Templars are outcasts and allying with them is forbidden. The Conclave thinks they can beat the Zerg conventionally but this is totally delusional because there are way way too many zerg. Your predecessor (who was believed dead) returns. He convinces you to side against the Conclave and ally with the Dark Templars because that is The Way to beat the Zerg. This involves protoss-on-protoss violence. In the end, you kill a bunch of Cerebrates and the Conclave admits its mistake. You attack the Overmind (with the help of Raynor) and your predecessor sacrifices his life to kill it. This works.
Campaign IV
You're still the same person, now again allied with what little is left of the Conclave. The Overmind is dead but you're still fucked because the zerg don't fall over dead; they're no longer strategic but still attack everything; besides the Cerebrates can control them as well. You flee to the home world of the outcast Dark Templar through a WARP GATE. Millions of Zerg follow you. Kerrigan shows up, saying she's no longer evil because she's no longer controlled by the Overmind (who is now dead). The guy from the conclave is having none of it but he's no longer the authority; an ultra old ultra respected Dark Templar called The Matriarch believes her. There's this ancient temple from the Xel'Naga who created the protoss and Zerg that has the power to wipe the planet free of Zerg. Kerrigan helps you activate it. The guy from the conclave rebels against you but you beat him. When you talk it through afterward, Kerrigan shows up and kills him. She tells you that she was just BSing you about being decent, using you to help her fight the Cerebrates. You proceed to use the Temple and kill every zerg on the planet. Meanwhile a new Overmind is born on the zerg homeworld
Campaign V
The UNITED EARTH DIRECTORATE (UED) shows up out of literally nowhere. You're the highest officer after the the leader DuGalle and his vice admiral and childhood friend Stukov. DuGalle first wants to get rid of the "Emperor" Mengsk. Some guy called Duran from the remains of the confederacy joins you. You successfully attack Mengsk's palace and cause him to flee (the UED has a lot of military strength and isn't incompetent), but Raynor shows up out of nowhere and saves him, then protoss show up and save both of them. Whatever, the UED still controls the human worlds now. You get hold of some important device that takes away the ability of Zerg leaders (New Overmind/Cerebrates/Kerrigan) to control the Zerg. You use it to attack the home world of the Zerg. You kill the relatively small group of zerg that they still control and enslave the Overmind using Drugs. The UED now controls most zerg forces (the rest are controlled by Kerrigan).
During this, Duran has done some clever manipulating to convince the DuGalle that Stukov was conspiring against him. DuGalle orders him to kill Stokuv, which he does. With his last words, Stukov convinces DuGalle of what really happened, but he can't get his hands on Duran who just fucked off. So DuGalle accidently killed his super loyal childhood friend (oops), and Mengsks isn't dead, but otherwise the mission was a success.
Campaign VI
You're the same Cerebrate from Campaign II, now obeying Kerrigan, who controls a sizeable minority of Zerg (most are controlled by the UED). Duran is also there, now obeying Kerrigan. The UED's device that takes away ability to control zerg is causing problems for her. She rallies Raynor and Mengsk and some protoss guy to help are, arguing that she's the only one who has the slightest chance to oppose the UED, which seems obviously true. Together you manage to destroy the device. Guarding this by half their army was literally the only thing the UED needed to do not to lose control, but alas others tend to become less competent when you're not the one controlling them. As payback for his help, Kerrigan promised Mengsk to destroy the UED forces on the planet where he was once emperor. She does this, he goes back to calling himself Emperor.
Once this is done, she betrays her friends, killing (a) the protoss guy who helped her and (b) Duke. She lets Mengsk live out of a weird sort of sadistic pleasure, considering him to be no threat to her now that Duke is dead.. She also doesn't kill Raynor for reasons not made explicit, but mentions that he's much smarter than Mengsk. Duran was in on it.
You go to the planet-of-the-dark-templars-now-protoss-homeworld and kidnap the Matriarch. A couple of other Dark Templar lead buy a guy called Zeratul follow you to rescue her. Kerrigan says she wants them to kill the Overmind for her, and then she'll let the Matriarch go. The Matriarch tells Zeratul to accept this offer. Zeratul kills the Overmind, but the Matriarch says she doesn't want to go back... because she's already been infested by Kerrigan! (Actually since before the start of Campaign IV.) Zeratul tries to kidnap her and forcefully get her back to the homeworld. You prevent this. Zeratul kills the Matriarch because that's better than her being a slave to Kerrigan. Kerrigan is impressed by this and lets Zeratul live so he can agonize over what he's done forever.
On the way back home, Zeratul receives signals of protoss life on some unknown moon. He investigates and finds a lab experimenting with Protoss and Zerg DNA. At the very end is a protoss zerg Hybrid creature, still in a state of coma but presumably going to wake up one day. Duran shows up. Zeratul asks if this is Kerrigan's doing but Duran says it goes way beyond Kerrigan's limited abilities or something and that he's some kind of ancient creature who's repeatedly interferred with history over the millenia (probably a Xel'Naga?) and that he's already put the Hybrid on tons of worlds and Zeratul won't ever find them all. He also says that the hybrid, which looks like an "abomination" to Duran, represents the pinnacle of both races or something. Disturbed, Zeratul goes back.
With the death of the Overmind, the zerg run rogue and the forces of the UED on the zerg home planet die quickly. The remaining UED forces, Mengsk, and the remaining protoss forces launch a surprise attack against Kerrigan, now alone as Duran has disappeared. She beats them. DuGalle commits suicide and the remaining UED forces are wiped out by the Zerg. Mengsk and the protoss go back to their home worlds. Kerrigan is effectively the ruler of the universe.