As for the Triwizard Tournament, it consists of three very dangerous tasks completed by the champions -- four champions, in this case -- throughout the school year. Alongside fellow Hogwarts champion Cedric Diggory (Robert Pattinson), Durmstrang champion and Quidditch player Viktor Krum (Stanislav Yanevski), and Beauxbatons champion Fleur Delacour (Clémence Poésy), Harry faces down a dragon and has to find a golden egg, goes underwater in Hogwarts' Great Lake to retrieve hostages from the merpeople inside, and makes his way through a maze filled with dangerous creatures like a sphinx and plenty of Blast-Ended Skrewts.
For most of "Goblet of Fire," one enormous and obvious question looms: who the heck put Harry's name into the Goblet in the first place? The object is enchanted so that you can only enter your own name, and Harry is too young to compete anyway. So how did this all even work?
The answer requires a lot of context. When Harry and Cedric reach the end of the maze together (injured but otherwise fine), they decide to claim the Triwizard Cup together and clinch the victory for Hogwarts. What they don't know is that the Cup has been tampered with and is a Portkey, a form of magical transport, which whisks them to a faraway graveyard. There, Harry comes face to face with Peter Pettigrew, who kills Cedric with a quick curse and uses Harry's blood to resurrect Voldemort (played his full corporeal form by Ralph Fiennes). Voldemort tries to kill Harry, but Harry's wand acts of his own accord and fights the Dark Lord off using specters of those he's killed -- including Harry's own parents -- giving Harry time to grab Cedric's body and the Cup and return to Hogwarts.
At first, Harry thinks that his Defense Against the Dark Arts professor Alastor "Mad-Eye" Moody (Brendan Gleeson) is behind the trap involving the Goblet and the Cup -- largely because, in the book, Mad-Eye just openly confesses. There's more to the story, though. The real Mad-Eye has been imprisoned all year, and Voldemort's loyal Death Eater, Barty Crouch Jr. (David Tennant), has been using Polyjuice Potion to impersonate him the whole time, get Harry into the tournament, get him to the graveyard, use his blood to revive Voldemort, and kill him.