The book opens with a long history of Tom Riddle Sr. (Voldemort's father), and his parents, who were found murdered one day in their stately manor. The Muggle villagers and the police suspect Frank Bryce, the Muggle caretaker of the house, to be the killer. As there is no clear evidence against him, he is released, but over the years, he is shunned by the villagers who do not believe his innocence. The scene where Frank, who still took care of the declining house all those years, discovers that there are intruders, is in the movie. The main differences are that Barty Crouch Jr. is not present, and Frank has a short conversation with Voldemort, in which he demands that he 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 terribly upsets the Dursleys.
The entire Quidditch game 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 destroying the camp site, Harry is not knocked unconcious as in the movie, but he notices that his wand is missing. He also has another run-in with Draco Malfoy. 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.
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 no avail) to convince her that House Elves are actually very happy to serve wizards and witches. This House Elf subplot is actually related to the first appearance of the Dark Mark, as in the book, Barty Crouch's house elf, Winky, is accused of casting the Dark Mark, and is fired from the family's service because of it. In reality, Winky was covering for Barty Crouch Jr. For the audience to be able to make sense earlier of who is behind this plot, the younger Crouch is seen both casting the Mark, and conspiring with Voldemort and Wormtail.
Harry and Sirius also correspond regularly in the book, rather than just once. He also meets Sirius in person shortly before the final task, and then again shortly after, when Dumbledore recruits both Sirius and Snape to begin recruiting and spying activities on behalf of the Order of the Phoenix.
An entire 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-natured 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: it is revealed that Bagman was brought to trial as a suspected Death Eater, but he was acquitted as he had not really joined their ranks. He is 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.
Barty Crouch Jr's existence is not revealed until the final part of the book. He is not mentioned until between the Second and Third Tasks, when Harry sees him on trial in the Pensieve. He is not open about his guilt as he is in the film and begs his father to show him mercy. He is assumed to have died in Azkaban (see above) and is eventually given the Dementor's Kiss accidentally.