So Star Trek Online is an MMORPG and has been around for a several years now. In the game there's a story, a convoluted one, but a story none the less.
Now imagine every single thing that has ever happened in the entirety of Star Trek regardless of tv show, movie, timeline, multiverse, animated show, whatever. everything. think of everything that has ever had "Star Trek" in it's title. got it? good. it's in the game. and in the story.
One aspect is the mirror universe which is a decent chunk of the story and it's a good excuse to bring back old Star Trek actors whose characters might be dead. Like there's A LOT of mirror universe stuff in the game.
really at the end of the day Star Trek Online is essentially the multiverse nexus of all Star Trek.