Harry, Ron and Hermione enter their fourth year at Hogwarts. After having a strange dream of the Dark Lord and his Death Eaters, Harry Potter awakes at the Weasleys' house where they shortly depart for the 422nd Quidditch World Cup. After the match, the camp is attacked by Death Eaters. The three later arrive at Hogwarts, where they find that the magical school will be hosting a legendary event called the Triwizard Tournament, in which one wizard each from two rival schools and Hogwarts will compete in three dangerous tasks. When the schools of Beauxbatons and Durmstrang, the rival schools, arrive, the rules are announced. Meanwhile, Alastor "Mad Eye" Moody arrives at Hogwarts at the request of Dumbledore to be the new Defence Against the Dark Arts teacher. Moody is a very eccentric man, noted for his paranoia and his wildly roaming "mad eye". People over seventeen who are interested must insert their name into the Goblet of Fire, and those chosen will be the Champions of their respective school. Three champions' names come out of the Goblet of Fire, Cedric Diggory from Hogwarts, Viktor Krum from Durmstrang and Fleur Delacour from Beauxbatons, and then the goblet unexpectedly produces a fourth, Harry Potter, although he is an under-age fourth year who never entered. Although Harry insists that he did not enter his name, his selection prompts a certain amount of jealousy and suspicion on part of his fellow students - including Ron, who believes that Harry is attempting to make himself the centre of attention again. This causes a brief period of resentment and estrangement between the two friends (including Ron swearing, telling Harry to piss off!), which is soon healed, however. Dumbledore tells Alastor to keep an eye on Harry, without him knowing it.
The Tournament begins with the first event, in which the four champions must each battle a dragon to retrieve a golden egg. Upon using the Accio spell to summon his broom, he flies out of the arena. The dragon breaks free of its chain and chases Harry through the Hogwarts grounds, where it runs into a stone bridge and falls dead. Harry gets the egg, which, when opened, will reveal a clue about the second task. When Harry opens it, though, only a horrible screeching is heard. They are soon informed of the Yule Ball, a Christmas ball held during the Triwizard Tournament. Ron and Harry find it hard to get dates to the ball, but finally get the Patil twins to join them, while Hermione goes with Viktor Krum, stunning everyone with her suddenly beautiful appearance at the ball. Ron is so jealously enraged at seeing Hermione with Krum (formerly his idol) that he attacks her and accuses her of "fraternizing with the enemy". The ensuing argument leaves Hermione in tears (in the film version only).
Cedric Diggory, the other Hogwarts champion, informs Harry to submerge the egg in water and open it. There he will hear the clue. Harry does as told in the prefects' enormous collective bath. The clue mentions that merpeople have taken something of Harry's and that they must retrieve it from the Black Lake. Just as the three are trying to find a way to breathe underwater, Ron and Hermione are called to McGonagall's office. Neville tells Harry about gillyweed, which, if eaten, can let you grow webbed hands and feet and gills. During the second task, Harry follows a mermaid to where they have chained Ron, Hermione, Cho Chang, and Fleur's sister. Cho Chang is retrieved by Cedric, Victor, the Durmstrang champion, takes Hermione. Harry takes both Ron and Fleur's sister, since she did not show up.
Finally, the third task arrives. A huge maze has been set up, in the center of which is the Triwizard Cup. The first to touch the cup wins the task. Dumbledore allows Harry and Cedric to be led into the woods first, because they both received first and second place in the previous task. Both Cedric and Harry grab the cup at the same time. The cup, which is actually a portkey, sends the two to a graveyard, where Wormtail appears with a form of Voldemort. Upon command, Wormtail kills Cedric and binds Harry to a tombstone. Wormtail performs a spell over a burning cauldron and drops Voldemort into it. A fully revived Voldemort emerges from it and summons the Death Eaters with the Dark Mark. After explaining to them how he lost his powers to Harry, he duels Harry Potter, in which their spells join in a chain. Harry breaks the chain and gets Cedric's body. After grabbing the cup again, he is transported back to the school where everyone learns that the Dark Lord is back and has killed Cedric. Mad-Eye Moody takes Harry to his office in order to help Harry calm down. Moody suddenly starts to change his appearance when he learns that he has run out of Polyjuice Potion. Moody tells Harry that it was he who told Hagrid to lead Harry into the woods and tell him about dragons. Moody also reveals that it was he who told Cedric to open the egg underwater and that he gave Neville the book that led him straight to discovering gillyweed in it. Harry realizes that Moody put his name in the Goblet of Fire and that he bewitched Krum. As Moody is about to kill Harry, Dumbledore, Snape, and McGonagall burst in the room and save Harry. Dumbledore makes Moody admit that he is an imposter and that the real Moody is locked in a chest. The imposter rapidly changes from appearing like Moody to his own appearance, Barty Crouch Jr, a Death Eater working for Voldemort. The tournament ends and the other schools depart.