Harry is in deep trouble. People at school are starting to think that he's the one that petrified a cat at the end of Chapter 8, and is also behind two more attacks. This time, a student and a ghost was petrified.
In the meantime, Ron, Hermione, and Harry have been using a derelict girls' restroom to concoct a potion that will transform them into anyone whose fingernail, hair strand, etc, is in the potion. The idea is expose another student as the real culprit. The reason that the bathroom is nearly abandoned is because a very whiny young girl ghost mopes about in there.
This novel has more ghosts than last time, and so far the ghosts don't seem to mature much beyond the point when they died. This grow older, but not maturer, was a factor in Cory Doctorow's Down and Out in the Magic Kingdom that didn't seem convincing. Would not a hundred years or more of fully cognitive experience lead to some self-restraint? I suppose if one's actions had few consequences, there would be little to learn from.
To complicate matters for Harry, the little house elf from many chapters ago, Dobby, has re-appeared. Dobby has powers of his own, his greatest one being annoying Harry. Harry finds out that Dobby somehow prevented him and Ron from entering platform 9 3/4, and later put a spell on a bludger. The bludger is an autonomic tool, in the quidditch game, whose sole purpose is to knock students off their flying brooms. Dobby's spell made the bludger charge after Harry to the exclusion of the other players. This eventually downs Harry, who falls and breaks his arm.
Enter Gilderoy Lockhart. He tries to mend Harry's arm with magic, but instead makes the broken bone disappear altogether. Lockhart's reputation as a master magician begins to falter, and accelerates downward in a disastrous dueling demonstration. During this demonstration, Harry's little-known talent is revealed to his friends and classmates: he can speak to snakes, and make them do things, like stop them from biting people.
If that's not creepy enough, he does so in a language known as Parseltongue, which is obvious to everyone but Harry. As far as Harry knew, he was speaking English to the snake. Take this skill, and Harry being present at all three victims' petrification, and his status as a suspect rises.
Harry, Ron, and Hermione are growing in their character roles. Hermione has an obvious crush on Lockhart, which Harry and Ron notice, but rarely comment on. Ron's sarcasm and ability to insult people is evolving, as well as Harry's ability to be mean. Yes, mean. Harry is downright nasty to Dobby, who doesn't beg much sympathy in the first place.
The three of them belong to a house of students known as Gryffindor. Their arch rival is the house of Slytherin, and its students. The Slytherin crowd are a nasty lot, but Harry and Ron are making their rebuttals more noticeable. What these kids can get away with is stunning, but their parents are not around, and teachers have a life, presumably. Their weekends, aside from quidditch, are essentially unsupervised.
Eleven-year-olds shouldn't require 24/7 supervision, but the level of misery and damage that they can inflict upon each other puts 6th grade gym class to shame.
The book is entertaining, and events are starting to accelerate nicely. Given the power that the students have now, at age 11, one wonders what they can do at age 16.