Though Overwatch takes place 60 years in the future, most of the world's important events happen decades before. Many of the game's heroes were around during both time periods, and have aged accordingly; that's why we have an assortment of grizzled old dads like Soldier 76, Torbjorn and Reinhardt. Some of them had a little bit more going in the intervening years, like Mercy (we'll get to her soon). And then there's Mei. By all rights, Mei should have hit menopause by now, but she still looks like an insufferable millennial. There's a good reason for that, and it might make you feel bad for hating her so much.
See, about 30 years before the game takes place, Mei-Ling Zhou was working in Overwatch's Anarctica station. As a scientist, she and her team were working to figure out the true cause of the planet's rapidly-worsening weather. They didn't get too far, because irony hit the base hard in the form of a massive polar storm. Life-sustaining supplies running low and no hope for escape, the scientists hopped in cryostasis pods as a last-ditch effort to survive. Every person at the station lay in hibernation, hoping that someone would arrive to help soon. That help never arrived.
Decades pass. The ruins of the station were finally rediscovered, and Mei was the only one whose cryo-pod was still functioning. Every other one of her colleagues had perished in what we can only imagine is a pretty hellish demise. One after the other, they likely woke up after the life support systems failed, alive long enough to realize they were trapped in their own pods, with nothing to do but gaze at the dead, or worse, those living that were still sleeping.
Mei awoke to a changed world, one without Overwatch and with an even worse climate than before. But she didn't let that get her down.
Maybe the most remarkable thing about Mei is how she seems to keep a positive outlook on the world, despite the horrible things that have happened to her. All of her co-workers died, and she missed out on what should have been the prime of her life. And yet, she still starts matches saying things like "The world is worth fighting for!" At heart, Mei just wants to make the planet a better place. Sometimes that means freezing a Genji and staring at him like a deranged serial killer before impaling his head with a giant icicle.
Overwatch isn't just your run-of-the-mill ragtag group of well-meaning vigilantes. They were originally assembled in an official capacity as a direct response to the Omnic Crisis. For a while there, humanity was living pretty peacefully with consumer-grade robots called the omnics.The original intent for these omniums was to "bolster manufacturing and create economic equality worldwide." But the company behind these robots, the Omnica Corporation, wasn't exactly on the level. When it was discovered that their plans included lies, fraud and other hornswagglery, Omnica was shut down and their 'bots were de-activated. The omnics were understandably a tad upset about this. Imagine if you were born with a great purpose, one that would help build paradise on earth -- and then suddenly everything was canceled and you were set for termination, just because your dad was kind of an asshole. That's what the omnics were going through.
Of course, because Terminator movies apparently don't exist in this world, nobody sees the violent AI uprising coming. The de-activated omnics went ahead and re-activated themselves, then began gathering arms by hijacking military equipment and repurposing giant mechs for what giant mechs were always meant to do: Fight everything.
Things were pretty dire for humanity. On a scale of "threats that might wipe out human existence," the Omnic Crisis was somewhere above the Cold War and just one notch below those KFC Double Downs with hot dogs inside. It was around this time that Overwatch came about, formed as a strike team to combat the omnics. This appeared to be working out pretty well, but the new squad had their work cut out for them.
A lot of it has to do with the fact that the omnics were built in these special factories called omniums. The Omnica Corporation had set these automated, self-sustaining robot mills up all over, and what was once convienent came back to bite the planet in the ass. Since the ominiums were designed to not only run themselves, but also constantly improve themselves and the robots coming off the assembly, this means the human-hating omnics in control of the operations had a never-ending supply of soldiers that were growing steadily more powerful. If you think Bastion was a real piece of shit before, omnium-ized Bastions basically have two Mercys while being shielded by three Reinhardts.
So why not just blow up these factories? Well, someone did, and that's how we got these dickbags:
Eventually, the Omnic Crisis ceased and an unsteady peace was declared. Australia in particular gave its omnics a swath of land to build on as part of the settlement. But the locals weren't thrilled with their new neighbors pushing them out of what was once their home. The Australian Liberation Front came together to fight against the omnics, and with the help of the man who would become Roadhog, they destroyed the main omnium on the continent. They didn't really think that part through. Omniums are fueled by fusion cores, and when these are destroyed, the surrounding land mass becomes a nuclear wasteland.
It was out of this Mad Maxian hellscape that a scavenger named Junkrat met a newly-deformed Roadhog, and the two went on to become international criminals. This is what you get when you don't play nice with robots.