Harry Potter and the Order of the Phoenix.

You-Know-Who is back and Harry has to fight him once more. Voldemort has figured out how to use Harry's mind and how to show him what he wants him to see. Harry learns why Voldemort is so against him and that his friends will always fight for him.

In Little Whinging, trouble arises. Dementors come after Harry and Dudley. Harry saves Dudley but got a letter from the ministry about a court case due to underage wizardry. This case will determine whether Harry Potter shall go back to Hogwarts.

The ministry is always looking for an excuse to expel Harry from Hogwarts ever since his 4th year when he told everybody Voldemort is back. The ministry believes in what it wants to believe.

Harry is not expelled this year, due to Albus Dumbledore. 

After that incident, Harry is moved to Sirius Black's house. Number 12 Grimmauld Place. The Order of the Phoenix is located there. The Order of the Phoenix is an organization made by Albus Dumbledore used to destroy Voldemort.

When Ron, Hermione, and Harry return to Hogwarts, Harry is known as a big joke. People won't take him seriously due to the whole, "Voldemort coming back" business.

Dolores Umbridge is the new Defense Against the Dark Arts teacher. She is from the ministry and will only teach them by reading. She later becomes head master and ruins the school.

Harry, Hermione, and Ron start this club where Harry can teach the Defense Against the Dark Arts called, "Dumbledore's Army." That's how Dumbledore got overthrown by Umbridge. They think he started an illegal club (Dolores had all these rules about how clubs need to approved).

Later, they head off (Ron, Hermione, Neville, Ginny, Harry, and Luna) because Harry thinks Voldemort has Sirius trapped there. But this was all just a big fat lie. Sirius was never there. Voldemort figured out how to use Harry's brain!

The Order gets involved, they have a battle against Voldemort. Sirius dies... and he vanished behind the black veil (the place where wizards go when they die).

Harry later learns that Trelawney made a prophecy about a boy born in late July who will overthrow the Dark Lord. And that boy was Harry Potter. "Both can't live while the other survives," is what the prophecy said. That's why Voldemort thinks he needs to destroy Harry.

What happens next? Read more in Harry Potter and the Order of the Phoenix by J.K. Rowling!