Simply put, in the final episode of Season Five of Buffy the Vampire Slayer, the villain created a giant portal that, if unchecked, would fuse every universe into one giant conglomeration. Buffy stops it by killing herself before it can do so, but before she does, a dragon escapes. In this timeline, that dragon is Nicol Bolas, and he plans on using the portal to gain Infinite Power from the multiversal collapse and in so doing regain his former status as a godlike energy being, like he was before the Mending nerfed the power of the Planeswalker Spark. So, he teleports Buffy into the ocean with a glance (to prevent her death from closing the portal), and sets about absorbing the Infinite Power.
Then Q, Oma Desala, the Wizard Marshall Zelretch (from the Nasuverse), and Luna (from the RPG Exalted) show up, having sensed the dimensional disturbance, and promptly get into a scuffle before the first two escort him to higher planes of existence to discuss things with other nigh-omnipotent beings, while Luna and Zelretch try to contain the dimensional damage and more and more entities show up to help them. Primus, from Transformers; Ed Greenwood, Elminster, Raistlin, and Mordenkainen from ADnD (there was a story in an old Dragon magazine about them all regularly meeting for tea at Ed Greenwood's house); Princess Celestia from My Little Pony: Friendship is Magic, and so on and so forth. Nyarlthotep showed up to troll everybody, but didn't actually help overly much.
They find that while the merging is too far gone to entirely reverse, they can prevent the more ludicrously powerful universes from merging (like Gurren Lagann, Dragonball Z, Asura's Wrath, the Culture, the Xeelee, and so on and so forth), and destabilizing things too much. They also find that they can prevent the universe from imploding into a point of infinite under the weight of all of the infinite variations of every person ever simultaneously existing by selecting the "stongest" (which is to say, most canonical) timeline from those of the universes merging into it, and in the case of those with no real dominant timeline (like Mass Effect or Fate/Stay Night), it selects from the best ending for each character (which was a contribution by Princess Celestia, and resulted in the occasional bit of strangeness like both Paragon mShep and Renegade fShep simultaneously existing). Additionally, they prevented the merging from wreaking massive geological destruction by causing those worlds with divergent geography (like most fantasy worlds) to integrate based on thematic similarities and available landspace; for instance, Rokugan wound up in China since Japan is too small, and Khorvaire wound up in North America, while Xendrick fused with South America.
In orbit, things are simpler; Luna had already eaten or seduced and absorbed every possible Lunar god in the entire possibility of Lunar gods, and that cut down on alternate moons, though the Earth still wound up getting a dozen extra moons and a planetary ring from Khorvaire, along with the Mi-Go's hiveship from Cthulhutech. Settlements were rather more confused, as the Fen suddenly found themselves sharing the moon with people from a bunch of different Gundam Franchises, Transhuman Space, and Eclipse Phase (along with Luna's assorted subservient lunar spirits); the folks on Mars suddenly find themselves an extreme minority as they suddenly find themselves amongst over three hundred million people, most of which come from Eclipse Phase and Star Trek. Those inhabiting relatively minor asteroids do better, though some degree of fiction is likely on more major asteroids like Pallas, which is pretty much a colony of Transhuman Space's Chinese, and liable to be rather confused about why the surface of their asteroid suddenly has an atmosphere.
And amidst all this chaos, the Fen are alternating between "Oh fuck, an alien invasion, and at least three different varieties of killer robot hordes", "OMIGOD I COULD ACTUALLY MEET PICARD", and "With all these different transfictional Earth governments around, we need to take action to prevent war from breaking out over who is really in charge".
Then Q, Oma Desala, the Wizard Marshall Zelretch (from the Nasuverse), and Luna (from the RPG Exalted) show up, having sensed the dimensional disturbance, and promptly get into a scuffle before the first two escort him to higher planes of existence to discuss things with other nigh-omnipotent beings, while Luna and Zelretch try to contain the dimensional damage and more and more entities show up to help them. Primus, from Transformers; Ed Greenwood, Elminster, Raistlin, and Mordenkainen from ADnD (there was a story in an old Dragon magazine about them all regularly meeting for tea at Ed Greenwood's house); Princess Celestia from My Little Pony: Friendship is Magic, and so on and so forth. Nyarlthotep showed up to troll everybody, but didn't actually help overly much.
They find that while the merging is too far gone to entirely reverse, they can prevent the more ludicrously powerful universes from merging (like Gurren Lagann, Dragonball Z, Asura's Wrath, the Culture, the Xeelee, and so on and so forth), and destabilizing things too much. They also find that they can prevent the universe from imploding into a point of infinite under the weight of all of the infinite variations of every person ever simultaneously existing by selecting the "stongest" (which is to say, most canonical) timeline from those of the universes merging into it, and in the case of those with no real dominant timeline (like Mass Effect or Fate/Stay Night), it selects from the best ending for each character (which was a contribution by Princess Celestia, and resulted in the occasional bit of strangeness like both Paragon mShep and Renegade fShep simultaneously existing). Additionally, they prevented the merging from wreaking massive geological destruction by causing those worlds with divergent geography (like most fantasy worlds) to integrate based on thematic similarities and available landspace; for instance, Rokugan wound up in China since Japan is too small, and Khorvaire wound up in North America, while Xendrick fused with South America.
In orbit, things are simpler; Luna had already eaten or seduced and absorbed every possible Lunar god in the entire possibility of Lunar gods, and that cut down on alternate moons, though the Earth still wound up getting a dozen extra moons and a planetary ring from Khorvaire, along with the Mi-Go's hiveship from Cthulhutech. Settlements were rather more confused, as the Fen suddenly found themselves sharing the moon with people from a bunch of different Gundam Franchises, Transhuman Space, and Eclipse Phase (along with Luna's assorted subservient lunar spirits); the folks on Mars suddenly find themselves an extreme minority as they suddenly find themselves amongst over three hundred million people, most of which come from Eclipse Phase and Star Trek. Those inhabiting relatively minor asteroids do better, though some degree of fiction is likely on more major asteroids like Pallas, which is pretty much a colony of Transhuman Space's Chinese, and liable to be rather confused about why the surface of their asteroid suddenly has an atmosphere.
And amidst all this chaos, the Fen are alternating between "Oh fuck, an alien invasion, and at least three different varieties of killer robot hordes", "OMIGOD I COULD ACTUALLY MEET PICARD", and "With all these different transfictional Earth governments around, we need to take action to prevent war from breaking out over who is really in charge".