system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Metal Gear Solid (The whole darn story)
gpt
Added a tl;dr at the bottom...

The Boss was a hero during WWII. Naked Snake (later Big Boss) was one of millions inspired by the Boss. The Boss became an undercover agent (essentially) in Russia, but her gift to Volgin (an American rocket launcher) that he fired started political tension between America and Russia. The Boss was told to scapegoat by the government and did so. Big Boss killed her because that was his mission (and, as it turns out, hers).

Big Boss found out the government threw the Boss under the bus and became disillusioned. About the country and about a soldier's place in it. Eventually, he defected to erect his own soldier state... all while still masquerading as a loyal American.

Big Boss saw all the problems after that stint in Russia and started a group with the other members of the operation in MGS3 called the Patriots, funded by a collection of money pooled between China, the US, and Russia after WW2. Part of the Patriots activities included cloning Big Boss (the perfect soldier), which resulted in two things: the new Snakes (Solid, Liquid, Solidus), which was the final straw for Big Boss. He quit the Patriots, while Zero assumed command, eventually began running the US by his whims, and once he became too old, had AIs run the country in his place. Think of the Patriots AI as the ultimate Big Brother; an all seeing eye that watches everything but also controls it.

Liquid's mission, ultimately, in MGS1 is simply to obtain Big Boss's genetic information, because, as a side effect of being clones, Liquid knew he'd eventually deteriorate (much like Snake did in MGS4).

Solidus, as the president, was well aware of the Patriots, however, and wanted to put an end to them. Solidus was an antihero in a way, as while his methods were extreme, he wanted to restore freedom to the world by destroying the Patriots. He failed.

Ocelot, acting as a triple agent (spying on the government spying on the Snakes spying on the Patriots, basically) took up the cause of eliminating the Patriots, acting as a resurrection of Liquid Snake to throw off the Patriots from his ultimate goal, allowing him to take control of the system of AIs. They recovered the AI from MGS2 and used it as their gateway into the main AI as a means to destroy it. Snake destroyed the AIs.

TL:DR; In a nutshell... Big Boss becomes disillusioned with the government and helps start the Patriots. The Patriots clone him (Liquid, Solid and Solidus Snake) and he becomes disillusioned with the Patriots and quits. He does a bunch of shit that on the surface makes it look like he's defected but is ultimately trying to destroy the Patriots, who have grown so immense in power since he left that they rule the entire world. From there, more or less everything in the Metal Gear Solid series has the ultimate goal of ending the Patriots, whether the participants know it (Ocelot) or not (Snake).

Hope that somewhat helps. But it's hard to accurately boil down such a rich yet convoluted story down to a few paragraphs.

And thanks for the gold!