Book starts with harry fighting at private drive where he and his cousin dudly are attacked by dementors and harry produces
patronus charm becuase of which he is expelled from hogwarts. Once again
dumbledore comes to rescue harry and harry spends half of his summer at the Black family home, 12 Grimmauld Place with sirius black, Hermoine and weasly family. He gets to know that to fight with
Voldemort the society called Order of Phoenix is Exist. The friends return to Hogwarts, meeting Luna Lovegood on the train. Hagrid has not returned to the school, and Dolores Umbridge, Senior Undersecretary to the Minister, has been appointed by the Ministry to become the new Defence Against the Dark Arts teacher and evntually takes contorl of school. Harry forms D.A. which becomes dumbledores army to teach Defence against dark art to others and because of this dumbledore needs to resign from school. Amazing and scary things continues to happen in harry’s life in which harrry loses his godfather sirius black and learns that there is a prophecy about Lord voldemort and Harry. At the end, dumbledore tells why voldemort wanted to kill harry when he was just baby – because of the prophecy which said,
The one with the power to vanquish the Dark Lord approaches...born to those who have thrice defied him, born as the seventh month dies...and the Dark Lord will mark him as his equal, but he will have power the Dark Lord knows not...and either must die at the hand of the other for neither can live while the other survives...the one with the power to vanquish the Dark Lord will be born as the seventh month dies... - Meaning harry has power to finish voldemort and one of them has to die at the end.