Alternate realities and other worlds are nothing new in the world of comic books. DC has the multiverse, where an infinite number of Earths exist, each with parallel versions of Superman, Batman, and all of the other major players in the world of DC comics. Marvel has the Ultimate universe, which serves as a way to retell the classic stories in a separate, but parallel world to the 616 universe, which is the "normal" Marvel world. There was also the Age of Apocalypse miniseries, in which Apocalyspe, in an altered reality, controlled the world, and Magneto was the leader of the X-Men. However, in most alternate reality storylines, the ending seemed to undo everything. The characters remembered nothing of the other world when things went back to normal. Such is not the case in House of M.
Wanda Maximoff, also known as Scarlet Witch, is the daughter of Magneto. She and her brother Pietro, also known as Quicksilver, are mutants, and thus have lived a life of oppression at the hands of humans. It doesn't help that their father is a maniacal terrorist bent on making the world a mutant paradise. Wanda's powers to alter reality have served her well as an Avenger, but lately, they have begun to take their toll on her mind. Prof. Charles Xavier has tried to help her, but she is quickly losing her grip on reality, which means that she could unknowingly alter the reality of everyone around her. Concerned for the safety of the world, Prof. X asks the X-Men and the Avengers to help decide how to deal with the problem. Fearing that Wanda's death is imminent, Piertro asks Magneto to do something, and suddenly, in a bright flash of light, everything changes.
Peter Parker (AKA Spider-Man) wakes up to hear his young son crying, and his wife Gwen Stacey (who died years ago in the 616 timeline) asking him to take care of it. Scott Summers and Emma Frost are married, Hank McCoy never took the serum that made him grow blue fur, The Kingpin has been taken out, Dr. Doom doesn't need to wear his mask, and Wolverine is a member of S.H.I.E.L.D. along with Rogue, Nightcrawler, Mystique, and Toad. And he has all of his memories of his life before the adamantium was added to his skeleton. Furthermore, mutants are the dominant species on the planet, and they oppress homo sapiens, and Magneto is the king of the world. While a few humans are respected, such as Tony Stark, Peter Parker, and Victor von Doom, most are terrorized by mutants, and human criminals are hunted down by sentinels. Operating out of Genosha, which is a sprawling metropolis as opposed to a desolate wasteland, Magneto and his children are the leaders of the free world. Every person that Wanda knew has had their greatest wish come true. So why is Wolverine suspicious?
Eventually, a series of events is set into motion that allows the heroes to remember the truth while living in the lie. While everyone is upset that Magneto used the Scarlet Witch to take over the world, some are more affected than others, especially Peter Parker. In this reality, he is a celebrity photographer, married to his first love. His Uncle Ben never died, and he knows nothing of the tragedies that he went through in the true reality. He nearly snapped upon learning the truth. By the end of the series, things occur in the Marvel universe that will have lasting effects for years to come.
Nearly every major character in Marvel's history makes some sort of appearance, either in this series, or a number of the tie-in series. Sadly, my favorite X-Man, Gambit, was only in one scene, but oh well. The story is very good, and it will be interesting to see how things turn out in the near future. The art is also great, and it makes the book as much fun to look at as it does to read it.
The series is steeped in backstory, so this is definitely not for beginning comic book readers (unless you're like me and you know people who can fill in the gaps for you). If you haven't read House of M yet, prepare to be blown away.