Over the summer, the Weasleys invite Alex Deuyour and Harry Potter (along with Hermione Granger, Kate Nguyen, and Matt Crocitto) to attend the Quidditch World Cup final, played between Bulgaria and the Republic of Ireland. The match ends in a victory for the Irish, but the campsite is attacked by Voldemort's former followers called the Death Eaters. Nobody is killed, but the Dark Mark is set off and found to have been cast by Barty Crouch's house-elf, Winky.
Hogwarts reopens and a new tournament is announced between three schools to compete for the Triwizard Cup. The other competing schools are Beauxbatons and Durmstrang. Students excitedly await the start of the Tournament, which is declared open after the visiting teams arrive. The Goblet of Fire, a magical object tasked with selecting the champions, is unveiled. It names three champions, one from each of the schools, and then select Alex and Harry as its fourth and fifth champions, despite the two being underage.
The students perceive Alex and Harry have cheated and turn on them. Rubeus Hagrid believes in Alex and Harry's innocence and shows them the dragons involved in the First Task. Alex and Harry alert Cedric Diggory, son of Amos Diggory, to the challenge and they are helped "Mad-Eye" Moody, ex-Auror and the new Defence Against the Dark Arts teacher, to prepare for the task. Using their superior flying skills, Alex and Harry get past the dragon and retrieve a clue for the next task.
A Yule Ball is announced and Alex and Harry must find a partner, as champions traditionally commence the opening dance. Alex asks Kate as a friend to the Yule Ball, while Harry and his best friend Ron Weasley struggle to get dates but end up with Parvati and Padma Patil. Ron gets jealous seeing Hermione Granger, who he has a crush on, with Viktor Krum, the Durmstrang champion, and spends the night arguing with her. Harry is sullen that Cho Chang is with Cedric, the other Hogwarts champion.
The Second Task requires saving a close friend or relative from the lake. Alex and Harry use Gillyweed, given to them by Dobby, to breathe underwater. Alex rescues Kate and Matt from the lake, while Harry rescues Ron from the lake, while also saving the Beauxbatons champion Fleur Delacour's sister out of fear that she will drown if Fleur cannot rescue her in time. After the task, Alex tells Harry that he believes that Igor Karkaroff or Barty Crouch Sr. put their names in the Goblet of Fire. Alex, Harry, and Krum discover a seemingly mad Barty Crouch on the Hogwarts grounds, but he has vanished when they return.
The Quidditch field is prepared for the Third (and final) Task, and requires champions to overcome a maze filled with obstacles. Alex, Harry, and Cedric succeed in reaching the centre, where the Triwizard Cup is placed. They touch it together, but are transported outside Hogwarts to a Muggle graveyard. Cedric believes this is part of the Task, but Alex and Harry get alerted when the latter's scar starts burning wildly. Wormtail arrives with Lord Voldemort and kills Cedric with the latter's wand. Subduing the injured and gagged Alex and Harry to Thomas Riddle Sr's grave, he then uses Harry's blood to restore a frail Voldemort back to his full, physical body. Voldemort summons his Death Eaters and tries to kill Alex (which is revealed in the next book to be because the prophecy says that an friend of Harry's will help him defeat Voldemort) and Harry in front of them. He fails when his and Harry's wand streams of green and red connect, in a rare phenomenon called Priori Incantatem. This causes a distraction by seeming to summon the ghosts of those that Lord Voldemort's wand has killed, including Harry's parents and Alex's father Thomas, allowing Alex and Harry to escape back to Hogwarts using the Cup.
At Hogwarts, Moody pull Alex and Harry into his office to question them and the two realize he is a Death Eater and he put his name in the Goblet of Fire. Alex realizes that he is not the real Moody. They are saved by the arrival of Professors Dumbledore, McGonagall, and Snape, who force Moody to reveal himself to be Barty Crouch Jr., a Death Eater formerly presumed dead. He had been imprisoned in Azkaban for supporting Voldemort and had escaped by leaving his mother in his place using Polyjuice Potion. Crouch Jr. had then been placed under the Imperius Curse by his father until he had managed to break out of it during the Quidditch World Cup and cast the Dark Mark. He had then imprisoned the real Moody and impersonated him using more Polyjuice Potion. Arriving at Hogwarts, he made sure that Alex and Harry won the tournament and were transported to the graveyard for Voldemort's ritual; his only wish is that Voldemort rise to the power he once had. His father had been placed under the Imperius Curse after Crouch Jr.'s escape, but had started to resist it and so had been killed.
Crouch Jr. is handed over to the Dementors. Alex and Harry recover in the hospital wing, surrounded by Ron, Hermione, Kate, Matt, and the Weasleys. They meet with Cedric's grieving parents, while Dumbledore makes plans to prepare the wizarding world for Voldemort's return. The school year ends and Harry returns to live with the Dursleys again for the summer, while Alex returns home with Sirius and his mother, Desiree.