Harry Potter and the Methods of Rationality is a Harry Potter fan fiction - No! Wait! Don't leave! Just hear me out. Please. I swear this is worth your time.
Like I was typing, Methods of Rationality is a fan fiction written by popular blogger and AI researcher Eliezer Yudkowsky. Ever the rebel against conventional wisdom, Yudkowsky chose fan fiction as a medium for telling stories.
Methods of Rationality appears to begin as a single point of divergence what if fic. Harry's aunt Petunia married an Oxford Professor and Harry grew up in a supportive home that fostered a love of reading, science, and critical thinking. As the plot progresses other points of divergence show up but the setting and characters remain mostly unchanged.
Very minor spoilers ahead
Harry James Potter-Evans-Verres's (adopted son of Petunia Evans-Verres and Michael Verres-Evans) life takes a drastic turn with the arrival of a letter of acceptance from Hogwarts. His "father" insists that Petunia's claim that her sister was a witch is absurd but she remains steadfast that magic exists. Rather than becoming involved in the argument Harry decides to actually ask for a demonstration and with the help of his neighbor, Mrs. Figg, sends a letter to Hogwarts. Professor McGonagall arrives and clearly demonstrates that the laws of physics are more normative than absolute. After Harry updates his world view he embarks on the wondrous adventure of discovering the wizarding world.
His response to Diagon Alley is more Elder Scrolls than Narnia. His first reaction to finding out about his fortune in Gringotts is to ask what the exchange rate between gold and silver is. From their he decides that he needs to diversify his holdings and maybe try his hand at arbitrage since the muggle and magical economies seem completely disconnected. After that he takes as much gold as he can and goes on a shopping spree that involves getting a mokeskin pouch and buying every item that he can think of a use for. He follows that by buying a trunk that includes a basement and can grow legs to follow him. After Harry and his father load the chest with a couple hundred maths, science, and sci-fi books he heads to King's Cross Station and departs for Hogwarts. The story continues with Harry intentionally and/or accidentally short circuiting the plots of the next six books and reaching the end of his quest by the end of the first year using his intelligence, knowledge, and genre savvy. Along the way he manages to create one or two new spells, fight in a three way mock war reminiscent of Ender's Game (this is not lost on Harry), and delivers a finishing move that would make mortal combat proud. The story ends on a note of cautious optimism which keeps with its general tone and somehow avoids using a Deus ex Machina despite putting Harry in what looks like an impossible situation.
Before I go any further I need to admit that I never read the original books, so these are comparisons to the movies. As such it falls to better read folks than myself to do a thorough comparison to the original series. That said I can offer some major differences. Ron is almost completely cut out of the story, getting a hand full of mentions. He is replaced by Draco who is given a more nuanced treatment than his mean kid status from the movies. The complex relation between Harry's attempts to make Draco reason and examine his beliefs rather than just accepting his father's politics and Draco's attempts to make Harry think a like a Slytherin generates a lot of character growth and tension throughout the story. Dumbledore also behaves differently, taking a Fizban approach to being a mysterious old wizard rather than the standard Gandalf. It's an open secret that his madness exists to obfuscate his genius, unless he really is senile and losing his mind. The question of how much of his zaniness is an act remains an open question for most of the story with Dumbuldore providing plenty of evidence for both sides. Last but not least is Professor Quirrell. His stutter is replaced with a fugue state and his Defence Against the Dark Arts class is actually about defending one self against other wizards instead of the usual collection of creatures from the D&D Monster Manual.
For whatever it's worth I consider Yudkowsky to be a competent author when he isn't brilliant. Most of his characters are three dimensional, the plot manages to carry half a dozen threads at a time with out becoming unwieldy, and the subject matter is informative as well as entertaining. That said, his work probably isn't for everyone. He writes his stories to illustrate concepts and I think it is fair to say that this is an author tract, all be it a balanced one. Yudkowsky puts his characters through the Ideological Turing Test which means that the villains typically argue quite persuasively even when they are obviously wrong. This is not a story with an easily accessible moral and it isn't meant to be. But if you are of a temperament that will put up with complexity and ambiguity (or even welcomes it) then you may find Harry Potter and the Methods of Rationality to be worth the time it takes to read.
Weighing in at 122 chapters and 661,619 words that's no small investment. It was completed on Pi Day and can be found on fanfiction.net or on its own site.