Harry Potter has a nightmare in which a Muggle caretaker is murdered after overhearing a plot by Lord Voldemort, Peter Pettigrew and another man whom Harry does not recognize. The following morning, Harry and Alex Edgbert attend the Quidditch World Cup with the Weasleys, Hermione Granger, Kate Nguyen, Matt Crocitto, Cedric Diggory and his father Amos. That night, Death Eaters attack the campsite after the tournament and the man from Harry's nightmare casts the Dark Mark.
At Hogwarts, Professor Dumbledore announces that the school will host the Triwizard Tournament along with the Durmstrang Institute from central Europe and the Beauxbatons Academy from France. One student from each school is to be selected by the Goblet of Fire to participate; students below the age of seventeen are ineligible. The Goblet selects Fleur Delacour for Beauxbatons, Viktor Krum for Durmstrang, and Cedric for Hogwarts. It then selects Alex and Harry as fourth and fifth Champions, respectively, causing much confusion. Many students believe Alex and Harry cheated and Ron shuns them, hurt that Alex and Harry did not inform him when they apparently entered.
For the First Task, the Champions have to collect an egg by getting past a dragon. Professor Moody, the new Defense Against the Dark Arts teacher, hints that Alex and Harry can use their wands to summon their broomsticks. All five Champions collect their eggs, which Alex and Harry do at the same time, because they are in the same House. Ron reconciles with Alex and Harry after seeing how dangerous the First Task was. On Christmas Eve, the school hosts the Yule Ball – Harry and Ron are unable to go with their desired dates, and hence go with Parvati and Padma Patil respectively; while Alex goes with Kate as friends and Hermione goes with Viktor Krum, to Ron's jealousy. Cedric advises Alex and Harry to use the Prefects' bathroom to get a clue for the Second Task using the egg.
For the Second Task, the Champions have to save somebody of value to them from the Black Lake: Alex has to save Kate, Harry has to save Ron, Cedric has to save his girlfriend Cho Chang (whom Harry tried asking to the Ball), Viktor has to rescue Hermione, and Fleur has to save her sister Gabrielle. Neville Longbottom gives Alex and Harry gillyweed to help them breathe underwater. Alex and Cedric come in first place after coming in at the same time and Harry is awarded second place after he saves not only Ron but Gabrielle after Fleur withdraws from the task. Alex and Harry later find the lifeless body of Barty Crouch Sr., a Ministry of Magic official, in the Forbidden Forest. In Dumbledore's office, they enter a Pensieve and witness the previous trial of Igor Karkaroff, current headmaster of Durmstrang, during Voldemort's first downfall. Karkaroff is asked to name those who served Voldemort – he gives Severus Snape who is vouched for by Dumbledore. Karkaroff then exposes Barty Crouch Jr., the son of Crouch Sr. who is in charge of the trial. Harry recognises Crouch Jr. from his nightmare.
For the Third Task, the Champions must navigate a maze to reach the Triwizard Cup at its center. Alex, Harry, and Cedric reach the Cup only to discover it is a Portkey that transports them to a graveyard. Peter Pettigrew kills Cedric on Voldemort's orders. He then uses Harry's blood to resurrect Voldemort, who summons his Death Eaters. Voldemort attempts to use the Killing Curse on Alex and Harry but they deflect it –the ghosts of Voldemort's previous victims appear, distracting Voldemort long enough for Alex and Harry to use the Cup to return to Hogwarts with Cedric's body.
Alex and Harry inform Dumbledore of Cedric's murder and Voldemort's return. Alex and Harry are escorted by Moody to his office – they learn that Moody entered them into the tournament and was guiding them to ensure the return of Voldemort. Before Moody attempts to kill Alex and Harry, Dumbledore, Snape and Minerva McGonagall subdue Moody. Using Veritaserum, they learn that they have caught Barty Crouch Jr. who was impersonating Moody using Polyjuice Potion; the real Moody is imprisoned in a magical trunk. Crouch Jr. is returned to Azkaban.
At the end of term feast, Dumbledore announces that Cedric was murdered by Voldemort, although the Ministry denies these claims. Alex and Harry inform Dumbledore of their encounter with Voldemort and Dumbledore describes it as Priori Incantatem. The three schools bid farewell to one another with Alex, Harry, Ron, Hermione, Kate, and Matt agreeing that everything is going to change.