Harry slams face first into the ground and stays still, clutching the cup and Cedric's body. After a few seconds, someone turns Harry over and he looks up at Dumbledore. Harry lets go of the cup and tells Dumbledore that Voldemort is back. Fudge tries to pry Harry's hand off of Cedric's wrist, but Harry doesn't let go until Dumbledore puts Harry on his feet. People scream and Harry hears Dumbledore tell him to stay. Moody, however, steers Harry away and asks what happened. Harry says Voldemort returned and killed Cedric.
It's a red flag when Moody leads Harry away against Dumbledore's orders; this shows the reader--and Dumbledore--that Moody isn't who he says he is and doesn't actually respect Dumbledore's authority. However, the fact that Harry is willing to talk to Moody right now shows that he's in no state to make this connection and believes that Moody is still his trustworthy professor.
In his office, Moody gives Harry a hot potion and things come into focus. Harry talks a little more about Voldemort's potion, but then remembers that there's a Death Eater at Hogwarts. Harry wildly asks if the Death Eater is Karkaroff, but Moody says that Karkaroff fled. He admits that he, Moody, put Harry's name in the goblet. As Moody points his wand at Harry, he asks how Voldemort treated the Death Eaters who escaped Azkaban. Harry listens incredulously as Moody explains how he guided Harry and easily manipulated Cedric and Dobby into helping. He admits to putting Krum under the Imperius Curse in the maze and Stunning Fleur. Harry watches over Moody's shoulder as the figures in his mirror get sharper.
Moody's note that it was easy to manipulate Cedric and Dobby suggests that there are dangers to being as kind and as giving, as both Cedric and Dobby were. Going forward, this will show Harry that he needs to be cautious and carefully evaluate offers of help, as not all of them will be well-intentioned. The fact that Harry struggles to process and believe all of this speaks to the success with which Moody made Harry believe he was kind and trustworthy.
As Moody says that he's going to kill Harry, Dumbledore, Snape, and McGonagall blast open the door and Stun Moody. Looking at Dumbledore, Harry understands why Voldemort fears him--he looks terrifying. McGonagall tries to take Harry, but Dumbledore insists that he needs to stay so he can understand. He tells Harry that this man isn't Moody, and sends Snape to fetch a Truth Potion as well as Winky. He then tells McGonagall to take the dog in Hagrid's pumpkin patch to his office. After they leave, Dumbledore begins unlocking Moody's trunk. When he uses the seventh key, he reveals a pit with the real Moody inside, unconscious. Dumbledore slips inside and points out the real Moody's hair--the imposter is using Polyjuice Potion. Harry and Dumbledore watch as "Moody" transforms into Barty Crouch.
The fact that both Snape and McGonagall comply with Dumbledore's seemingly strange requests without question impresses upon the reader how trustworthy Dumbledore is, especially since, in Harry's mind, McGonagall and Snape are working on opposite sides. When Dumbledore discovers the real Moody inside his own trunk, it shows that Sirius was right in suspecting that someone tried to stop Moody from getting to Hogwarts--in other words, there is truth to be gleaned from Rita Skeeter's articles.
Snape and McGonagall return and Winky throws herself at Barty. Dumbledore administers the Veritaserum to Barty, revives him, and asks how he escaped from Azkaban. Winky cries as Barty says that he and his mother used Polyjuice Potion to switch places with each other. She remained in Azkaban and died there, and Mr. Crouch staged his wife's death at home. Barty was kept under the Imperius Curse and an Invisibility Cloak and nobody but Bertha Jorkins knew he was still alive. Mr. Crouch altered her memory to make her forget, but the spell ruined her memory.
When Barty Crouch reveals that Mr. Crouch messed up Bertha Jorkins's Memory Charm, it suggests that the emotional nature of the situation compromised his father's abilities to properly perform the spell. This shows the reader that for all Mr. Crouch's faults, he did love his son and wanted to protect him, even if his methods were questionable and ultimately failed.
At Dumbledore's prodding, Barty says that Winky talked Mr. Crouch into letting him see the Quidditch World Cup. He and Winky sat in the top box all day, and he was already learning to fight the Imperius Curse at that time. He stole Harry's wand and when the Death Eaters began destroying tents, it made him angry. Winky bound Barty to her and pulled him away from the Death Eaters, but he still sent the Mark into the sky. The Ministry wizards Stunned both of them, and Mr. Crouch discovered him and fired Winky.
Barty's testimony reveals that Winky was struggling to walk through the woods because she was magically dragging Barty, not necessarily because she didn't have permission. This shows that though Winky failed in her attempt to follow through with Mr. Crouch's instructions, she did her very best and actually was capable of acting independently in a way she thought best.
Then, Barty says that Voldemort arrived, having discovered from Bertha Jorkins that he was still alive. Voldemort put Mr. Crouch under the Imperius Curse and sent Barty to take Moody's place so he could get into Hogwarts. Voldemort and Wormtail remained at the Crouch home and eventually, Mr. Crouch began to fight the Imperius Curse. He finally escaped and when he arrived at Hogwarts to confess, Barty saw him on the Marauder's Map and killed him. Finally, Barty says that he carried the cup into the maze earlier and turned it into a Portkey.
Barty Crouch's mention of the Marauder's Map shows the reader and Harry again that they should take magical objects seriously, given that the reader now knows that Harry actually saw Barty Crouch in Snape's office, not Mr. Crouch. This again impresses upon Harry the importance of drawing on his world to come to accurate conclusions.