As was the case in the previous adaptations, subplots have been removed, separate events compressed and combined, and certain scenes drastically simplified to accomodate the movie's running time. The most important deviations from the book are listed below:
The book opens with a long history of Tom Riddle Sr. (Voldemort's father), and his parents, who were found murdered fifty years before in their stately manor. The Muggle villagers and the police suspected Frank Bryce, the Muggle caretaker of the house, to be the killer. As there was no clear evidence against him, he was released, but over the years, he was shunned by the villagers who did not believe his innocence. Only the scene where Frank, who still took care of the declining house over the years, discovers that there are intruders, is in the movie. The main differences is that in the book, Barty Crouch Jr. is not present; Voldemort only mentions an unnamed faithful servant, and that he killed someone called Bertha Jorkins. Upon detection, Frank demands that Voldemort face him like a man; Voldemort replies he is no man, but complies anyway. The "thing" that Frank sees is not described, but it causes him to scream in fear before he is killed by the green flash of the Killing Curse.
The movie then cuts to Harry waking up at the Weasleys' residence. In the book, however, he wakes up at home with the Dursleys. He gets a letter from Ron who tells him they will come and pick him up for the Quidditch World Cup. However, they use the Floo Network and enter through the fireplace, which causes an unbelievable mess and terribly upsets the Dursleys, since their fireplace is an electric-powered fireplace and was boarded up. Harry goes with the Weasleys to their home, meeting the older brothers Bill and Charlie, and learning that Fred and George have plans for opening a Joke Shop (over their mother's objections).
In the Quidditch stadium, Harry meets Winky, a House Elf, who says she is saving a seat for Mr. Barty Crouch, her master. Most of the Quidditch match was omitted from the movie for time reasons. In the book, Victor Krum grabs the Snitch, but this cannot prevent Ireland from winning with a 10-point difference.
While the Death Eaters are causing a stir, Harry is not knocked unconcious as in the movie, but he notices that his wand is missing. He also has a run-in with Draco Malfoy who warns him that the Death Eaters are coming for the Mudbloods. Harry does not see the person who conjures up the Dark Mark (as later revealed, this person was hidden under an Invisibility Cloak), he merely hears the incantation "Morsmordre" being spoken. Unfortunately, Aurors locate Harry's wand at the place and confirm that it was used to create the Mark, so they have an even more legitimate reason for suspecting Harry of conjuring the Mark than in the movie. Then Winky is also found there, unconscious, unable to explain her presence. Crouch is furious and fires Winky from the family's service because of her disobedience. She is employed at Hogwarts' kitchen later on. In the movie the Death Eaters chase the entire population out while destroying the camp site, but in the book they quickly Disapparate as Aurors arrive at the scene.
Harry and Sirius also correspond regularly in the book by owl post and Floo network, rather than just once. Sirius resides with Buckbeak in caves around Hogsmeade, and he meets Harry personally in Hogsmeade shortly before the second task. It is revealed later that he also remains in close contact with Dumbledore.
The personality of Barty Crouch Sr. has been changed a bit for the film. Though strict and bureaucratic, he appears relatively friendly and compassionate. In the book, he is portrayed as uptight, relentless and quite brutal, especially in his methods for fighting the Dark Arts; he even sent Sirius to Azkaban without a formal trial. Crouch Sr. seems to care only about the success of his career in the Ministry and was at one time a candidate for becoming the Minister of Magic; however, his ruthless attitude towards his own son (a Death Eater) prevented him from ever attaining the post. After the first task, Crouch gets ill and isn't seen again until just before the third task, his place being taken by his aide, Ron's older brother Percy. In the movie, Crouch attends all activities himself.
Several classes are given in the book that were omitted from the movie, such as Care for Magical Creatures classes from Hagrid (where the students learn how to raise quite dangerous Blast-Ended Skrewts) and Divination classes (as always filled with grim predictions from Professor Trelawney that never come true). During further Defense against the Dark Arts classes, Moody teaches students how to resist the Imperius Curse.
There is an extended subplot involving Rita Skeeter, who publishes articles in the newspaper. However, she has the tendency to twist stories, put quotes out of context, and present gossip as fact, causing many wizards to be slandered and embarrassed, and Harry to become increasingly unpopular among the students. For instance, she discredits the Ministry of Magic with a story about the disappearance of one of their workers, Bertha Jorkins (no one had made a point of this previously, as Jorkins is known for her extreme forgetfulness). Hagrid is accused of being a Giant, creatures generally considered hostile by the wizarding world, causing him to hide in shame and him being substituted by Professor Grubbly-Plank, until Harry, Ron and Hermione convince him there is nothing to be ashamed about. Hermione becomes a target of hexed hate mail after an article accuses her of dating both Harry and Victor Krum. It is long unknown where Skeeter gets her information from, as listening devices don't work within Hogwarts. Skeeter's role in the book also set up the plot for the following book through a article that cast Harry in a bad light, causing Harry to be deemed as an unreliable witness to Voldemort's return.
While at Hogwarts, Hermione makes numerous issues about how the House Elves are treated at Hogwarts and in general. To improve their lives, she creates the S.P.E.W. - the Society for the Promotion of Elvish Welfare, in spite of the fact that everyone attempts to convince her that House Elves are actually very happy to serve wizards and witches (to no avail). This subplot was never used in the films.
The Goblet of Fire is a wooden cup in the book. The Beauxbatons' girls all seem to be Veela, meaning they have the ability to seduce nearly every male at school, and their Headmistress Maxime is thought to be at least half-Giant like Hagrid (which she denies heavily).
In order to taunt Harry, Draco starts a hate campaign directed toward him while declaring Cedric the legitimate Hogwarts champion. Tensions rise, leading to a duel between Harry and Draco, during which both Goyle and Hermione are hit by stray curses. Snape intervenes, sending Goyle to the hospital wing, but, in typical Snape fashion, trivializing Hermione's injuries. Ron and Harry loudly object, which (also predictably) leads to points deduction and detention. Mr Ollivander, the wand maker, makes a short appearance in a Wand Weighing ceremony where the champions' wands are given a check.
Harry and Hermione make a trip to Hogsmeade, with Harry under his Invisibility Cloak in order to avoid more snide remarks. They meet Hagrid and Moody in The Three Broomsticks, and find out that Moody's magical eye can penetrate his cloak. Hagrid invites Harry over in order to show him what the First Task is about. In the forest, Harry notices that both Madame Maxime and Igor Karkaroff are there, so they can warn their respective champions. Since there is no one for Cedric, Harry decides to warn Cedric himself.
While figuring out the clue to the second task, Harry notices on his Marauder's Map that Barty Crouch is inside professor Snape's office. Which is most curious, as Barty Crouch Sr. was said to be ill. Harry hides under his Cloak as Filch and Snape come dangerously close to discovering him, but he is saved by Mad-Eye Moody, who can see him but does not betray his presence to the others. Noticing Harry's map afterwards, Moody asks if he can borrow it. Harry agrees, and asks why what Mr Crouch was doing in Snape's office. Moody suggests that Crouch was seeking evidence that Snape still practices dark wizardry (but we learn at the end that the Barty Crouch that Harry saw walking inside Snape's office was in fact Crouch Jr. disguised as Moody, stealing ingredients for his Polyjuice Potion).
Dobby the House Elf returns in the book but was omitted from the movie. He gives Harry Christmas presents and is the one providing him with the Gillyweed. We later learn that fake Moody (Barty Crouch Jr. in disguise) tried to teach Neville Longbottom about Gillyweed, hoping that he would pass the information to Harry. But when Neville did not act as expected, he "leaked" the information to Dobby instead. In the movie, the book's first suggestion is followed, and Neville provides Harry with the Gillyweed.
Another character that was omitted from the film is Ludo Bagman, an ex-Quidditch player working for the Ministery of Magic. He is introduced during the Quidditch World Cup chapters as a gentle and good-nature man who is very partial to gambling. Throughout the book he tries to give Harry some advice about the tasks to come, not in the least because he is betting heavily on Harry and hopes this information will help Harry winning. Later on, he becomes one of the suspects who could have put Harry's name into the Goblet of Fire, as Harry witnesses one of Dumbledore's memories in the Pensieve: this reveals that Bagman was brought to trial as a suspected Death Eater. However, due to his naive nature, he had merely passed information to an acquaintance, not knowing that the man was a Death Eater, so he is quickly acquitted. Bagman is also involved in a lengthy subplot with Fred and George, who bet all their savings on the outcome of the Quidditch World Cup and won, but were paid in leprechaun gold (which vanishes eventually). It is eventually revealed he had his gold taken by a group of goblins to whom he was in debt, and so bet on Harry to win the Triwizard Tournament, offering him illegal advice wherever possible.
In the movie, Crouch Sr. is found dead just prior to the Third Task, but in the book, Harry and Victor Krum find him walking the grounds, deliriously asking to see Dumbledore. Harry goes to get Dumbledore, but as they return, Crouch is gone and Krum claims that Crouch Sr. stunned him before escaping.
Before Harry's look in the Pensieve, he has a dream during Divination class of Voldemort torturing Wormtail for "almost ruining everything". He then proceeds to Dumbledore's office to tell about the dream. There he arrives when Dumbledore and Fudge are arguing over the mystery of Crouch Sr.'s disappearance, and Fudge suspects Madame Maxime to be the culprit due to the event occurring near the Beauxbatons's carriage. However, Dumbledore suspects this accusation comes from Fudge's personal prejudice, since Madame Maxime is a half-giant. In the film however they argue about whether to cancel the tournament. After Fudge and Dumbledore leave, Dumbledore's Pensieve also shows the first appearance of Bellatrix Lestrange (although not identified by name), a character mentioned in the previous book (her first appearance on film is in the next movie). She and her husband are extremely devoted Death Eaters who, unlike most of the others, proudly boast their allegiance and loyalty to Voldemort and make no effort to hide their crimes, which earns them a life sentence in Azkaban prison (where the husband eventually died).
Barty Crouch Jr.'s presence is not revealed until the final part of the book (in the movie we see Crouch Jr. making the Dark Mark, and he appears in Harry's vision). He is merely mentioned as having died at Azkaban, and not seen until Harry's peek in Dumbledore's Pensieve. In that memory, he has already been caught as a Death Eater and is brought to trial together with Bellatrix and her husband. In the book he largely maintains his innocence (contrary to the film), even begging his father for leniency. Dumbledore comments that Crouch Jr's involvement was probably a mix of naivety and consorting with the wrong people; nevertheless, even though his guilt was never unquestionably proven, he was given a life sentence (in the movie, there was plenty of evidence for his guilt). Dumbledore also reveals that Neville's parents were tortured into insanity by Bellatrix, and are permanently living in a magical hospital.
The maze in the Third Task contains a lot more dangers, such as Blast-Ended Skrewts, magical illusions, Boggarts and a Sphinx; Harry saves Cedric from a giant spider which injures his leg, prompting Cedric to share the victory by simultaneously grasping the Triwizard Cup.
Voldemort appears as something of a horrible cross between an infant and a snake before his resurrection to a full body. He is even more disappointed in the book with his Death Eaters than in the movie, intimidating and even using the Cruciatus Curse on some of them for their lack of loyalty following his downfall. He mentions all those who have died for him, specifically stressing the absence of the Lestranges in the graveyard, calling them his only true followers for not denouncing him when they were caught. He also mentions a coward follower (presumed to be Karkaroff), a follower who may never return (Snape), and a loyal follower at Hogwarts (revealed to be Crouch Jr.). He explains how he became a ghost after trying to kill Harry, forced to survive by possessing animals, until he was finally able to possess Professor Quirrell prior to Harry's first year. During the duel, the beams from the wands merge into one golden beam, and Harry and Voldemort are lifted up and placed elsewhere in the graveyard before the dome forms. Harry hears a Phoenix singing that he must maintain the wand connection; also, the Priori Incantatem effect causes a replica of Wormtail's silver hand and the ghost of Bertha Jorkins to emerge prior to Harry's mother.
The unmasking of Barty Crouch Jr. is more elaborate in the book. He explains under influence of Veritaserum how it was his mother's dying wish to see her son released from prison. So the two switched places in Azkaban using Polyjuice Potion, with Crouch Jr., leaving prison with his father, disguised as his mother. The mother soon died, disguised and buried as her son. Crouch Sr. kept his son hidden and controlled him by means of the Imperius curse, together with Winky, his house elf. Winky was supposed to keep an eye on Crouch Jr. (hidden under an Invisibility Cloak) during the Quidditch Cup, but in the chaos, she lost him out of her sight; he stole Harry's wand and created the Dark Mark. Prior to the Quidditch Cup, Bertha Jorkins, the disappeared witch, had found out about Crouch Jr. being alive, but she was captured and tortured for information by Voldemort and Wormtail, who learnt of the Triwizard Tournament and Crouch Jr.'s situation. They broke the curse, kidnapped Moody and replaced him with Crouch Jr. When Sr. found out about his son's escape, Wormtail kept him under the Imperius curse, stating he was ill. However, Crouch Sr. eventually broke the curse, escaped and tried to warn Dumbledore at Hogwarts, but his son saw him on the recently-procured Marauder's Map, stunned Krum and killed his own father, transfiguring the body into a bone (Harry witnessed Wormtail receiving punishment for this in his dream). The movie drastically simplifies this backstory: Crouch Sr. wasn't even aware that his son had escaped Azkaban; he was found dead after he discovered his son was impersonating Moody (due to the flickering of his tongue which Crouch Sr. recognized that only his son could do). The movie does not offer any explanation for his escape from Azkaban, only implying that he must have gotten out unnoticed.
In his office, Dumbledore explains that when the spells of two "brother wands" meet, one of them will be forced to recall the last couple of spells produced, called the Priori Incantatem effect. This happened since Harry and Voldemort's wands both contain a feather from the same Phoenix. Sirius later appears in Dumbledore's office as well, when Dumbledore orders both him and Snape to begin recruiting and spying activities on behalf of the Order of the Phoenix. The Minister of Magic comes to Hogwarts together with a Dementor to interrogate Barty Crouch Jr. further, but the Dementor immediately performs a Dementor's Kiss on Crouch Jr., leaving him for braindead and unable to attest to Voldemort's return. This is followed by a discussion between Dumbledore and Cornelius Fudge, the latter not believing Harry's claim that Voldemort has returned in corporeal form due to Rita Skeeter's article. This leads to a schism between Dumbledore and the Ministry of Magic, which features prominently in the next book and film.
Harry has a difficult meeting with Cedric's parents, who show him their gratitude for bringing back Cedric's body. Dumbledore urges Harry to go back to the Dursleys despite an invitation to stay with the Weasley family (the reason being explained in the next book). Hermione reveals that she found out that Rita Skeeter is an unregistered Animagus; she often listened in on others while disguised as a beetle (Harry did indeed notice the presence of a beetle during one of his overheard conversations). Hermione blackmailed Skeeter by threatening to report her to the Ministry of Magic, if she keeps publishing unsubstantiated gossip. The final chapter of the book reveals her interviews from Slytherin students such as Draco Malfoy and Pansy Parkinson which helped with these slandered facts was due to them knowing her secret and that they fed her these facts while in her beetle form due to Dumbledore having banned Skeeter from entering Hogwarts after the First Task. Draco, Crabbe and Goyle confront Harry, confirming Voldemort's return and making threats, but Harry responds by hexing them, together with Ron, Hermione, Fred and George. Harry gives the prize money from winning the Tournament to Fred and George, so they can finally open their Joke Shop.