In the beginning...there was a witch.
A Scarlet Witch, to be precise. One who lost her mind, and with the help of her brother, re-created the universe to her liking.
Much to her disappointment, others weren't as pleased with the new universe as she was, and attempted to force her to put it back as she'd found it. And so she changed it once more...
But she didn't put it back.
Thus, a new beginning. In the new reality, there was a man with a dream. His name was Charles Xavier, and he gathered together six mutant teens and trained them in the use of their powers. Their names were Scott Summers, Jean Grey, Warren Worthington III, Henry McCoy, Robert Drake, and John Proudstar. They were joined sometime later by Alex Summer and Lorna Dane, and became the X-Men, based in Westchester, New York. Together they fought against mutants who didn't share Xavier's dream of mutual co-existence, and protected those who hated and feared them. And Xavier was pleased.
In the meantime, on the other side of the Atlantic, another team was formed by a former colleague of Xavier's, Dr. Moira MacTaggert. Named Excalibur, they aided the people of the UK and became celebrities, much like the Avengers of the United States. Their names were Brian and Betsy Braddock, Meggan, Kurt Wagner, Katherine Pryde, Piotr Rasputin, Sean Cassidy and Logan. They were celebrated as Britain's Greatest Heroes, and Moira was pleased.
The teams first met when Moira received an unexpected call from her old friend. His X-Men had been captured by the sentient island Krakoa, and he sought the help of her team to rescue them. She agreed, and Excalibur set forth to beat the island into submission.
They succeeded, despite the tragic loss of John Proudstar. And the teams went their separate ways, keeping in touch but rarely interacting due to their respective responsibilities.
Years passed. Lorna and Alex left the X-Men. Brian and Meggan married and left to form an academy for mutants in Britain with Sean, while Eric Lenscherr opened one in America. Others joined both teams, and other teams, such as the government-sponsored X-Factor, came into existence. And while on a mission in outer space to combat the alien Brood, the X-Men and Professor Xavier encountered the Shi'ar. And the Professor fell in love with their Empress, Liliandra. Leaving Scott Summers in charge, he left with her to help her regain the throne usurped by her sister Deathbird, and the X-Men returned home to battle more terrestrial threats.
They never imagined that their greatest threat was from within.
Jean Grey's powers, long held in check by the Professor, spiralled out of control and her sanity was threatened. Calling herself Phoenix, Jean threatened to take over the world and set herself up as a goddess. Unable to stop her by themselves, the X-Men contacted their old friends in Excalibur. Together they fought the Phoenix, and brought Jean back to her senses. But at a cost.
While Excalibur was across the Atlantic, Piotr's ten year old sister Illyana disappeared. Despite the best efforts of both teams, no trace of her has been found.
It's now a month since the events of that night. Excalibur has been disbanded indefinitely, pending extensive investigation by the British government into the events of the Phoenix incident, and the two teams have banded together in Westchester. Moira is working to evaluate Jean's condition, and in the meantime...
Well, two teams contained within the same mansion, no matter how large, are likely to throw off a few sparks.
X-Men: Aberrations