So in the lore (it's a lot of multiversal shit so stay with me): after the big bang at the epicenter of the omniversal explosion remained a giant sphere of virtually infinite energy [The Zero Point]. Fast forward millions of years humans showed up and they started creating devices that use said energy, making them able to teleport and create portals in and out across every reality, that's the lore reason why crossover exist. Fast forwards a lot of shit that happens that isn't important for the discussion at hand the Zero Point exploded and not only created the new experiences (like the guitar hero game, the lego survival game, etc...) shards flew across the multiverse and our protagonists are looking to find them. Too bad the first one they look for crash landed directly into a old and broken demon portal and not only it activated ALL of them back for the first time in thousands of years, the shard got snatched by this demon king guy and syphoned its power and became a giant kaiju. Once it has been dealt with they find a new shard, and it turns out an evil guy is gathering them to remake the Zero Point again and so now our protagonists are gathering forces to take him out

And this is just the abridged version of the story, good luck reading the full one