At Hogwarts, Professor Dumbledore announces that Alastor "Mad-Eye" Moody will be the Defence Against the Dark Arts teacher for the year. Dumbledore also announces that Hogwarts will host a revival of the Triwizard Tournament, in which a champion of Hogwarts will compete against champions from two other European wizarding schools: Beauxbatons Academy, and Durmstrang Institute. The champions are chosen by the Goblet of Fire from names dropped into it. Because Harry is under 17 (the age of majority in the wizarding world), he is disallowed from entering.
At Halloween, the Goblet of Fire picks Fleur Delacour from Beauxbatons Academy, Viktor Krum, the Bulgaria seeker that played at the Quiditch World Cup, from Durmstrang Institute, and Cedric Diggory from Hogwarts to compete in the tournament. Unexpectedly, however, it also chooses Harry as a fourth champion. Despite anger from the Beauxbatons and Durmstrang contingents, Harry's being chosen magically binds him to compete, forcing him to be an unwilling participant in the Tournament. Very few people believe Harry's protests that he did not put his own name in; Ron, envious of Harry again being the centre of attention, accuses Harry of lying and breaks up their friendship.
In their first Defence Against the Dark Arts lesson, Professor Moody introduces the class to the Unforgivable Curses: the Imperius Curse, through which a wizard can be controlled as another wills; the Cruciatus Curse, which causes immense pain; and "Avada Kedavra", the killing curse.
A tabloid-style reporter, Rita Skeeter, starts writing scandalous articles of half-truths and outright fabrications in "The Daily Prophet" about those at Hogwarts, starting with Harry. Because of the Prophet's high readership, her words hold a lot of sway among the wizarding population.
Despite champions not being allowed to know, Hagrid covertly reveals to Harry that the first task is to get past a dragon. Madame Maxime, the headmistress of Beauxbatons, and Professor Karkaroff, headmaster of Durmstrang, also discover this. Sure that they will tell their respective champions, Harry informs Cedric about the dragons in the interest of fairness. Harry struggles to think of a way past a dragon, until Moody suggests flying. Hermione helps him to perfect a Summoning Charm, which he uses to summon his Firebolt broomstick and fly past the dragon to retrieve a golden egg, receiving high marks from the judges. Ron and Harry subsequently reconcile, Ron now understanding the full danger of the tournament. Harry's egg is supposed to contain a clue to the next task, but when opened it merely shrieks loudly.
Hermione infiltrates the school kitchen as part of her house elf welfare campaign (in which she is finding few allies and much resistance). She takes Harry and Ron there to meet Dobby, who now works there after his freeing in Harry's second year. They also find a distraught Winky, who is seriously depressed after being sacked. Dobby is the only known house elf to appreciate his freedom, despite his hardworking nature; the others reject Hermione's idea of payment and holidays, which they find distressing.
The students are informed of the Yule Ball, a Triwizard Tournament tradition, which requires Harry to find a partner with whom to open the dance. He eventually asks Cho Chang, a Ravenclaw fifth-year with whom he is smitten, but she has already agreed to go with Cedric. Ultimately, Harry asks Parvati and Padma Patil to go with him and Ron. Hermione, offended that Ron did not consider her except as a last resort, attends with Viktor Krum, causing intense jealousy in Ron. At the ball, Cedric tells Harry to take the egg into the bath in the prefects' bathroom to work out the clue, but Harry is reluctant to accept the advice because he is jealous over Cho.
Rita Skeeter writes an article revealing Hagrid's half-giant parentage (though it is a mystery how she found out), a taboo in the wizarding world. After Hermione confronts her, Skeeter writes articles painting Hermione as Harry's unfaithful girlfriend. After masses of hate mail, Hermione swears revenge on Skeeter.
The trio secretly meet Sirius at Hogsmeade, who tells them that Barty Crouch was fanatical about catching dark wizards, often stooping to the same level to punish them. This culminated in Crouch sending his own son to Azkaban. His son later died there. Sirius suspects that someone is attempting to attack Harry through the Tournament and make it look like an accident. He suspects Karkaroff, who is a former Death Eater who walked free.
Finally acting on Cedric's tip, Harry takes the egg to the prefects' bathroom by night, where with help from Moaning Myrtle he listens to the egg underwater. There, the shrieks become the words of a song, which tells Harry that the task is to recover something dear to him from Merpeople at the bottom of the lake in the grounds. While sneaking back to Gryffindor Tower under his invisibility cloak, he sees Barty Crouch in Snape's office on the Marauder's Map, despite Crouch supposedly being too ill to judge the Triwizard Tournament. While investigating, Harry falls into a trick step and drops the egg, which shrieks loudly and attracts Filch, Snape (who has had potion ingredients stolen), and Moody. Moody's magical eye sees Harry stuck in the stair under his cloak, but Moody covers for him, then borrows the Marauder's Map.
Harry, Ron, and Hermione are unable to find a way to survive underwater for the second task, leaving Harry panic-stricken. He is rescued on the morning of the task by Dobby, who gives him some Gillyweed that he heard Moody and Professor Sprout talking about. The Gillyweed gives Harry flippers and gills and allows him to find Ron at the bottom of the lake. However, he refuses to leave the hostages belonging to the other champions behind, and insists on rescuing Fleur's sister when Fleur does not arrive. Although Harry finishes last, the judges (apart from Karkaroff) still award him high marks for 'moral fibre.'
One month before the final task, Harry and Krum are talking near the Forbidden Forest when they encounter Barty Crouch, who stopped appearing to work at the Ministry several months ago. Although insane, in moments of clarity he confesses to have done 'something terrible,' that Bertha Jorkins is dead, and begs for Dumbledore. Leaving Krum with Crouch, Harry fetches Dumbledore but returns to find Krum stunned and Crouch gone, Krum claiming that Crouch attacked him from behind. Moody gives chase but does not find Crouch.
During a Divination lesson, Harry experiences another dream involving Voldemort punishing Wormtail for a 'mistake.' Harry tells Dumbledore about this, and stumbles upon a memory-keeping device in Dumbledore's office, a Pensieve. Inside the Pensieve, he discovers that Ludo Bagman was accused and acquitted of Death Eater activity, and that Barty Crouch's son was sent to Azkaban for supposedly helping to torture Neville Longbottom's parents into insanity.
Harry prepares for the final task, a hedge maze filled with dangerous creatures and obstacles, the goal being to reach the Triwizard Cup at the centre. Inside the maze, Harry stuns Krum, who was using the Cruciatus Curse on Cedric. Helping each other, the two reach the Cup. They agree to touch it at the same time and become joint winners. However, on touching it they discover that it is a Portkey that transports them to a graveyard. There, Wormtail appears, kills Cedric using Voldemort's wand, and ties up Harry. Wormtail uses Harry's blood, Tom Riddle Sr.'s bone, and his own hand to restore Lord Voldemort to a body.