Voldemort has returned, but The Minister of Magic refuses to believe so. Only Dumbledore and a few others at Hogwarts believe Harry, who saw Voldemort return himself. It is up to Harry and The Order (an anti-death eater group) to not only stop Voldemort from getting what he wants, but also convincing the Ministry and Ministry believers that Voldemort has in fact returned.
You see he was trying to tell Dumbledore that the weapon is ready
Harry keeps having repetitive 'dreams' about a door. During these 'dreams' he is somehow connected to Voldemort, who wants to get through the door, so Harry does as well. Whenever Harry awakes from these 'dreams' his scar burns, and he is angered that he did not see what is behind this mysterious door (Harry speculates that whatever is behind the door is the 'weapon' Voldemort wants to get his hands on).
The prophecy has been smashed!
Umridge (a new teacher the ministry has sent to work at Hogwarts and 'inspect' it) is slowly starting to take over the school and have it only abide by the ministry's rules and beliefs. She makes educational decrees that do not work in Harry's favor, and punishes everyone who goes against the Ministry. Soon enough she finds a way to kick Dumbledore out and becomes the Headmistress of Hogwarts.
I told you Voldemort has returned.
I guess he has...
Hermoine comes up with a quick plan to lie to Umbridge to get her off Harry's back for why he was using her fireplace to travel. She tells Umbridge that he were trying to tell Dumbledore the weapon was ready, and that the weapon is in the Forbidden Forest. Umbridge demmanded to see it, but while in the forest they came across Centaurs who carried Umbridge away after she had angered them.
Show me this weapon immediately!
Harry, Ron, Hermione, Ginny, Neville, and Luna (now able to escape Hogwarts) went to the Ministry to stop Voldemort and save Sirius (who was in a vision of Harry's where he was kidnapped). At the Ministry, they realized Voldemort was after a prophecy about Harry, and got into a battle with Death Eaters over it. Eventually, the prophecy got smashed and Voldemort appeared, very angry. A little after that Dumbledore appeared to fight off Voldemort.
The Dark Lord will not be happy!
Once the fight between Dumbledore and Voldemort had finished, the Minister of Magic had come just in time to see Voldemort. At first, he refused to believe what he saw but after a chat with Dumbledore, he could not deny any longer that Voldemort had returned.