Having broken the rules about using magic in front of "muggles" by saving his bullying cousin Dudley (HARRY MELLING) from an unusual attack by soul sucking fiends known as dementors, Harry Potter (DANIEL RADCLIFFE) learns that he's been expelled from the Hogwarts School of Witchcraft and Wizardry. While head instructor Professor Dumbledore (MICHAEL GAMBON) successfully defends Harry in front of a Ministry hearing held by Minister of Magic Cornelius Fudge (ROBERT HARDY), the teenage wizard ends up with a tarnished reputation among his classmates.
That is, except for longtime friends Ron Weasley (RUPERT GRINT) and Hermione Granger (EMMA WATSON). They're concerned about Harry's growing inner turmoil and angst, while new and somewhat ethereal student Luna Lovegood (EVANNA LYNCH) befriends him, and Cho Chang (KATIE LEUNG) continues to draw his romantic eye. More troubles are afoot, however, and they extend beyond Harry's rival classmate, Draco Malfoy (TOM FELTON).
For starters, a new Defense Against the Dark Arts teacher has arrived in the form of Dolores Umbridge (IMELDA STAUNTON). She's a strict disciplinarian who's soon issuing a plethora of decrees that restrict the students -- including stopping them from practicing magic -- while also putting school leaders Dumbledore and Professor McGonagall (MAGGIE SMITH) on notice. She also creates unease among the likes of Professor Snape (ALAN RICKMAN), Sybil (EMMA THOMPSON), and groundskeeper Rubeus Hagrid (ROBBIE COLTRANE), and with the backing of Fudge, she becomes the most powerful voice at Hogwarts.
Harry also learns of the secret Order of the Phoenix, populated by the likes of Alastor "Mad-Eye" Moody (BRENDAN GLEESON), Remus (DAVID THEWLIS), and his godfather Sirius Black (GARY OLDMAN), among others. Harry's parents were once part of the Order that's now concerned that the evil and all-powerful Lord Voldemort (RALPH FIENNES) is planning to return, build and army with the likes of Lucius Malfoy (JASON ISAACS) and Bellatrix Lestrange (HELENA BONHAM CARTER), and hopes to acquire a certain weapon to do his evil bidding. Learning from Snape that Voldemort's mind is connected to his own and could cause serious consequences, Harry must contend with various nebulous nightmares and visions related to that, all while dealing with the rest of what's occurring at his school.