There's so much to cover in this movie—close to 900 pages of Potter-y goodness—that it just hits the ground running. And so will we.
Harry and his cousin, Dudley are attacked by dementors almost right away. Harry manages to fight them off (and save the always-odious Dudley) but is then informed that he's expelled from Hogwarts.
Mo' magic, mo' problems. Er—make that mo' "using magic outside of Hogwarts property while underage," mo' problems.
Later, Harry finds out that he'll actually get a trial before being kicked out of school—some wizards come and whisk him away to a safe house in London. They're from the Order of the Phoenix, a secret society that Dumbledore formed back when Voldemort was in power. Harry also finds out that the Ministry of Magic has been covering up the truth about Voldemort's return. Crooked politicians exist even in the wizard world, we guess.
At Harry's trial, Dumbledore successfully defends our favorite boy wizard, the charges are dropped, and—surprise, surprise—Harry Potter returns for his fifth year at Hogwarts. There, he meets the new Defense Against the Dark Arts teacher, Dolores Umbridge. It's obvious that Umbridge and the Ministry of Magic don't want Hogwarts students learning any actual defensive spells.
When Harry protests that they might need to learn how to defend themselves against dark magic because, well, you know, Voldemort is back, Umbridge punishes him by making him carve words into his own hand with a torturous quill.
Yeah, she's not a nice lady.
Umbridge keeps ruthlessly grabbing power at Hogwarts until Hermione has the idea to form a secret club for practicing Defense Against the Dark Arts. Harry will be the teacher and they'll meet in the Room of Requirement. They call themselves Dumbledore's Army; these budding wizards mean business. Umbridge clearly knows students are up to something, but she can never catch them in the act, which forces her to take more and more desperate (and evil) measures.
While all this is happening, Harry keeps having weird dreams about a strange black hallway. One night, he sees Mr. Weasley being attacked by a snake—and, lo and behold, Mr. Weasley has actually been attacked. Turns out Harry has a weird connection to Voldemort's mind. One he needs to learn to shut off immediately. Good luck with that, kid.
Meanwhile, Dumbledore's Army is finally caught in the act by Umbridge. She rounds up the students and accuses Dumbledore of plotting to overthrow the Ministry. After all, they are called "Dumbledore's Army." Dumbledore takes the blame for everything to save his students from serious trouble, then escapes before he can be taken to Azkaban. That leaves Umbridge as headmistress of Hogwarts. Uh-oh.
One day, Harry sees a vision of his godfather, Sirius, being tortured by Voldemort. Unfortunately, he, Ron, and Hermione are caught trying to use the Floo Network in Umbridge's office. Hermione thinks quickly though and tells Umbridge that Dumbledore is hiding a secret weapon in the Forbidden Forest. There, Umbridge is attacked by a group of centaurs that she calls "filthy half-breeds."
She's a cruel, racist tyrant and we don't feel bad for her. Not even a little bit.
After ditching Umbridge, Harry and friends make their way to the Ministry of Magic but they don't find Sirius there. They do come across a mysterious orb with Harry's name on it. The orb turns out to be a prophecy about Harry and Voldemort. Huh. What a coincidence…
Just then, Death Eaters come out of hiding and demand the prophecy. Seems Harry was lured there with false visions from Voldemort. (Those will get you every time.) Harry and his friends manage to fight the Death Eaters off for a bit, but Harry has to hand over the prophecy once the Death Eaters take his buddies hostage. So much for their training with Dumbledore's Army.
Luckily, they're saved by the Order of the Phoenix who show up and start kicking some Death Eater butt. Sadly, Sirius Black is killed right in front of Harry's eyes by a Death Eater named Bellatrix Lestrange. Wow. Harry can't have anything nice, can he?
Harry tries to chase down Bellatrix, but Voldemort and Dumbledore show up and stage an epic wizard duel. We're talking jets of light, fire snakes, giant water balls, and broken glass everywhere. (We really feel bad for the janitor at the Ministry of Magic.)
In the end, Voldemort escapes the duel and lives to fight another day, but everyone at the Ministry sees him, so they can't pretend he's not back anymore. Harry also learns that he's going to have to kill Voldemort or else be killed by him. Bummer.
Then it's all aboard the Hogwarts Express until next year…when things will be even more complicated.