(Review contains slight spoilers. Read at own risk)
Harry's ready for his 4th year at Hogwarts, but as usual things are not as they appear. Wizards have been going missing and Voldemort's supporters are getting braver. This would seem like child's play if Harry didn't have a sneaking suspicion that someone was trying to kill him. When his name comes out of the Goblet of Fire and Harry is forced to participate in the deadly Triwizard Tournament he has to learn to juggle school work, friends, crushes and the constant threat of Voldemort.
Goblet of Fire marks a real change in the series. I know I said that about Prisoner of Azkaban, but stay with me. Goblet, at a hefty 700 pages is action pact. Previously Rowling has shown her audience a relatively normal year at Hogwarts before shoving as much important information into the last 100 pages as possible. Here the information is evenly paced, and there is so much happening that it's hard to get tired of any one storyline.
It is with Goblet that the series takes a darker turn. We witness our first real time death. The stakes are raised with Voldemorts supports getting braver and Voldemort himself making a brief, but very important appearance. Goblet also reveals a lot more information about the dark arts with the introduction of the unforgivable curses. Goblet is definitely the turning point in the series. It is here where the books become for a slightly older audience, and where the characters themselves really start to grow up.