Harry's fifth year story begins in the midst of the summer after the events of 'Goblet of Fire'. Spoiler alert , but not really, Voldemort has returned, and the wizarding world is divided. Most of the world is too afraid to admit that this could be true, and have therefore dubbed the truest believers to be nutters - Harry and Dumbledore being the main culprits.
However, a secret organization known as the Order of the Phoenix have Harry and Dumbledore's backs on it. Back when Voldemort was in power previously, the group were the main band of fighters who resisted Voldemort's power. Parts of the group include Mad-Eye Moody, the Weasley family, Professor Lupin, and Sirius Black just to name a few. The group works in the background of the story as they work on figuring out what Voldemort is up to - something involving a sort of "weapon" he hadn't had during his former reign of power.
Meanwhile, Harry, Ron and Hermione head back to Hogwarts, where the Ministry of Magic (the main governing body of the magical community, and the biggest representation of non-believers in the case of Voldemort's return) has decided to interfere. The annually open position for professor of Defense Against the Dark Arts is filled by the love-to-hate Deloris Umbridge (Imelda Staunton). She rather quickly works her way up the ladder to become Hogwarts' High Inquisitor, imposing a ridiculous amount of rules and regulations that pretty much prevent the students from having any enjoyment of school at all. She starts by banishing magical use from Defense Against the Dark Arts, and providing beginners texts, and proceeds to carry out some rather severe punishments if anyone steps out of line. Not even Dumbledore can do anything about it, though, as the Ministry has control of the entire situation.
Eventually, a rebellion led by Harry is formed so that anyone interested can actually practice real magic in order to defend themselves from whatever it is that's coming. That whole concept alone is what makes this one, one of my personal favorites in the movie universe. It's the first time there's a real threat that enough people know about, AND it's the young crowd taking matters into their own hands. It's pretty much the first time we see a collective group of students working together towards a common goal, be they Gryffindor, Hufflepuff or Ravenclaw. As for the Slytherins, they are too busy siding with the darkness, and being recruited by Umbridge to try to figure out what's happening behind her back.
I always personally found this story to be a bit underrated, as it tends to be the least liked of the books. But I've always had a thing for the new ideas that come into play here, namely Dumbledore's Army. But it doesn't stop there. In a big way, we don't see just the students working together, but the staff of Hogwarts too. Umbridge is consistently drilling them and making sure everything is up to snuff according to Ministry standards. We don't see an uprising from the teachers, but the idea lends itself to the concept of hating Umbridge even more. She's easily make my Top 5 of all time, as in "Characters I Love to Hate". It's interesting, but throughout the whole series, I would almost consider her the worst villain short of Voldemort himself.
The end result of the movie is satisfying, to say the least. As an adaptation, I'd have to say this is one of the best ones. It's got some of the best sets and character development throughout the series, and doesn't manage to leave too many important things out of the book. But I'll get into that next.
COMPARING THE BOOK
So, I guess there's no doubt some of my readers are asking themselves "But HOW could it be one of the best adaptations? The book is HUGE! They would have had to cut out a lot of important stuff, right?" Well... honestly, you may be surprised. Part of the reason this is one of the more disliked books is due to all the filler. It can be interesting, but none of it truly lends itself to the full story. There are bound to be a few exceptions, but in just about any case you CAN find a way around it with your imagination. Though it's perfectly understandable to not wanna have to do the work.
For a few gaping examples, I'd turn to a few sections of the book that take a fair amount of time to read for little tidbits of information. There's about three full chapters in the book that go between the snake attack on Arthur Weasley to the end of Christmas. The big set ups from this are to show Harry can see what Voldemort is up to, and to get Harry to Occlumency (blocking out thought intrusion) classes with Snape. The movie does a good job of getting all of the info together that's needed, and having it still make sense in a shorter amount of time - something any good film adaptation should do.
There are, though, a few things the movie cuts out unnecessarily that would have been nice to see. For instance, when it comes to Ron and Hermione, you might be wondering what happened to their respective stories from the book? They both actually become Gryffindor prefects. Hermione spends much of the time trying to get ready for her O.W.L.s, and helping Harry where needed. So there's not a HUGE gap for her. Ron, on the other hand, has a whole story about getting onto the Gryffindor Quidditch team. In fact, speaking of Quiddtch, it's sadly thrown right out of the movie. One of the biggest things that happens in the book is that Harry, Fred and George step out of line after a Quidditch match against Slytherin, banning them from Quidditch as long as Umbridge is there. Knowing hoe important Quidditch is to Harry, that would have lent itself in a big way to the hatred for Umbridge we all felt.
Perhaps the biggest adaptation faux pas for me was showing Percy Weasley working for the Ministry of Magic. There's a whole thing in the book about how Percy has gone against Harry and the rest of his own family by siding with the Ministry, and being a prat about it. He disbands from his own family, even sends Ron a letter saying how he shouldn't hang out with Harry or anyone who believes him on what he says. In the movie, he kinda just shows up in Dumbledore's office, has no dialogue, and to the fans who haven't read the books, this is downright confusing.
Further character cameos are also pushed from the film adaptation. Dobby is in this one again, and actually tells Harry about the Room of Requirement (where Dumbledore's Army meets and practices). Once again, the film essentially replaces Dobby with Neville in this role (the former being informing Harry of Gillyweed in 'Goblet of Fire'). There's also a cameo from Professor Lockheart, as he's still at St. Mungos Hospital from when Ron jinxed him in 'Chamber of Secrets'. He doesn't serve much of a purpose though, so cutting him was practically expected.
I'm sure there are other holes I'm missing here, but you'll have to forgive me for not remembering every detail of a 956 page book. The good news is that this was the biggest obstacle I knew I had to get through. If not for the thickness of the book, for the overwhelming and frankly irritating temper Harry has in the book (another thing that places this on the less-popular list). By the way, that might be the best thing the movie adjusts for the better. Harry is irritable, but not nearly as bad as he was in the book - a formula thankfully remedied in the proceeding stories!