Harry Potter dreams about an event while it takes place. He sees an elderly man investigate a light that is shining in the house he cares for and overhears Lord Voldemort making plans with Peter Pettigrew and another man that Harry does not recognize. The caretaker is caught and killed by Voldemort. Ron Weasley and Hermione Granger express concern over Harry's dreams. The Quidditch World Cup permits Harry a distraction from his nightmares. However, followers of Voldemort known as Death Eaters attack and destroy the spectators' campsites after the match.
At Hogwarts, headmaster Albus Dumbledore introduces a new teacher, a man known for his powerful magic and his all-seeing glass eye, the auror Alastor "Mad-Eye" Moody. Harry, along with his classmates, notices Moody frequently drinking from a flask. Dumbledore also announces that the school will host the Triwizard Tournament, in which one wizard from each of three schools competes in three difficult tasks. The champions are selected by the Goblet of Fire, a magical cup into which the names of candidates are placed. Cedric Diggory is chosen to represent Hogwarts, Quidditch champion Viktor Krum is chosen to represent Durmstrang Institute, and Fleur Delacour is selected to represent Beauxbatons Academy of Magic. The goblet unexpectedly chooses a fourth champion, Harry. As Harry is underage and therefore should have been ineligible to compete, Hogwarts teachers and fellow students alike are suspicious.
In their first Defense Against the Dark Arts lesson, the students learn of the three Unforgivable Curses. The Imperius Curse causes absence of free will, the Cruciatus Curse causes unbearable pain, and the final curse, Avada Kedavra, causes death.
In the first task, each of the four champions are instructed to retrieve a golden egg guarded by a dragon. Moody probes Harry about which talent he should use to overcome the dragon, and the two ultimately decide upon flying. Harry summons his broomstick and retrieves the egg, which contains information about the second challenge. In exchange for previous aid, Cedric Diggory provides him with a clue that prompts him to open the egg underwater. With help from Moaning Myrtle, he learns that the second task entails the retrieval of "something precious" to each of the competitors from mermaids in the nearby Black Lake. In last-minute preparation for the task, long-time friend Neville Longbottom provides Harry with Gillyweed, which allows him to breathe underwater. Harry is the first to arrive at the location, and finds Ron, Hermione, Cho Chang and Fleur's sister, Gabrielle Delacour, in suspended animation. Finishing last in an attempt to free all four at once, Harry is awarded second place for "outstanding moral fiber."
Ministry official Barty Crouch is found dead shortly after the second task. While waiting in Dumbledore's office, Harry's curiosity leads him to look into Dumbledore's pensieve, causing him to revisit one of Dumbledore's memories. He witnesses a trial before the Wizengamot (Wizard Court) in which captured Death Eater Igor Karkaroff, the current headmaster of Dumstrang, denounces a number of Death Eaters, including both Severus Snape and Barty Crouch Jr. While Dumbledore vouches for Snape's integrity, Crouch Sr. is horrified at this revelation and denounces his son, sending him to Azkaban. Upon returning to reality, Dumbledore tells Harry that he is searching his memories for a clue as to why extraordinary events have taken place at Hogwarts since the start of the Triwizard Tournament. While walking the halls Harry is confronted by Snape, who believes Harry has stolen the ingredients for Polyjuice Potion, by which a person can change their appearance.
In the Triwizard Cup's third and final task, the four competitors are placed inside a large hedge maze, the challenge being to reach the cup first. Viktor Krum, acting under the Imperius curse, incapacitates Delacour and attempts to do the same to Cedric. Harry stops Cedric from attacking Krum, and the two run for the cup. When Cedric is trapped by vines, Harry frees him and the two claim a draw and touch the cup together.
The Triwizard Cup transports the two champions to a graveyard where Voldemort is waiting for Harry. Peter Pettigrew kills Cedric and traps Harry. Pettigrew performs a ritual that rejuvenates Voldemort, who then summons the Death Eaters and bids them witness a duel between their lord and his nemesis. As Harry repels Voldemort's killing curse, a connection called Priori Incantatem occurs between their wands. Harry's wand forces Voldemort's to disgorge the spirits of the people Voldemort has most recently killed, including Harry's parents and Cedric. Harry is briefly protected by the spirits and escapes with Cedric's body using the cup.
Upon his return, Harry tells Dumbledore and Minister for Magic Cornelius Fudge that Voldemort has returned and is responsible for Cedric's death. Moody leads Harry back to the castle, where his questions make Harry suspicious. Upon the arrival of Dumbledore, Snape and McGonagall, Moody transforms into Barty Crouch Jr.; the Polyjuice Potion he kept in his flask wears off. The real Alastor Moody has been kept prisoner in a trunk.
As the representatives from Durmstrang and Beauxbatons depart, Dumbledore exhorts them to stand together against Voldemort, telling them that "Dark and difficult times lie ahead. Soon we must all face the choice between doing what is right, and what is easy."