It started with the darkest day in Avengers history (that actually still holds true to this day). The Scarlett Witch used her reality altering powers to make former and current Avengers to take out her current Avengers teammates. After Dr. Strange told the Avengers that it was Wanda powers making her crazy (well, The Wasp reminded her that she had kids that she conjured and weren't real and was supposed to forget is what really made her crazy), Dr. Strange stepped in and helped the Avengers take her down. But the Damage had been done. Clint Barton (Hawkeye) and Scott Lang (Ant Man) were killed fighting Wanda's creations.
Immediately after the fight ended, her father Magneto appears from a wormhole of his creation and takes her to Genosha in hopes that his friend Charles Xavier would help her get her powers under control. After trying over and over, Xavier realized that there was nothing he could do to help her. So Xavier calls the Avengers and the X-Men together to figure out the fate of Wanda Maximoff.
There was a lot of debate on what to do. Wolverine and Emma Frost agreed that the best thing to do would be to put her down before she could cause any more damage to the world. Their logic was that if Dr. Strange (The Master of Mysticism) and Professor X (the word's strongest telepath) couldn't help her, then there were no more options. The Avegners and some of the X-Men refused to believe that murder was the answer. After arguing, the X-Men and The Avengers decided that they should go to Genosha in hopes of talking to Wanda and finding a way to help her. But when they get to Genosha, she's not where she was last seen and then Xavier disappears. Emma uses her telepathy and finds him on another part of the island, but before the teams could get to Xavier or Wanda, the world goes white and is changed.
When the world comes back, Wanda had given all of her friends and other heroes everything they had always wanted. Carol Danvers was the most popular hero in the world, Simon Williams was a movie star, Peter Parker got to be loved as Spider-Man by the public and his Uncle Ben and Gwen Stacy were alive again, and to top it off, mutants were the dominant species. They were 95% of the world and the world was run by her father and former teorrist Magneto. Unfortunately, Wolverine also got what he always wanted as well. He got all of his memories back. On top of the world the way it was now, he got all of his memories of before the world changed and the ones that were wiped clean by Weapon X and others. After waking up to this world, Wolverine set out to find out what happened and how to change it back.
This is yet another great starting point for new readers. This really gets the ball rolling with all the events that follow. It's debatable that this is THE event that starts it all. After the government finds out what Wanda did to all the mutants of the world, it probably gave them a reason to start policing not only mutants, but people with powers in general. This leads right into "Civil War" which goes right into the death of Captain America and so on. The story itself is pretty damn good but has been met with mixed reviews.
I recommend this title for those of you who want to see the whole story of how the Marvel Crossover events became a yearly thing, but more so if you're a fan of the X-Men. The aftermath of "House of M" forces Cyclops to go from just the leader of the X-Men and disciple of Charles Xavier to the leader and protector of an entire race. He has to start thinking outside of the box. The teachings of Xavier are now obsolete and it's time for Scott Summers to take the reins and steer the ship the way he feels is right. It's no longer about coexisting. It's now about survival.