Harry, Ron, and Hermione cling tightly to the dragon. As they fly, Harry wonders how long it'll take Voldemort to realize what happened. Hours later, the dragon starts to descend toward a lake. The trio jumps into the water and watches the dragon continue to the far bank. They tend to their burns, inspect the cup, and laugh at their ridiculous predicament. Suddenly, Harry's scar bursts with pain and he finds himself in Voldemort's mind, torturing a goblin and learning that Harry stole a cup from the Lestrange's vault. Voldemort screams in rage that Harry discovered his secret. He murders the goblin and others, and wonders if Harry and Dumbledore know his secret. He decides to check on his other Horcruxes, and thinks he'll visit Hogwarts last and keep Nagini close to him.
Harry's excursion into Voldemort's mind shows him Voldemort's biggest weakness: that he believes himself to be smarter than anyone else, and invincible because of that. In other words, Voldemort has spent his life devaluing everyone else's intelligence, especially those he despises like Dumbledore and Harry, and in doing so he's left it wide open for someone else to figure out that he has Horcruxes. His desire to check on his Horcruxes suggests he might understand this mishap, while it's also important to remember that he has the Elder Wand and, therefore, he likely still feels invincible.
Harry returns to his own mind, tells Hermione and Ron what he saw, and says they need to go to Hogwarts immediately. He says that Voldemort is going to warn Snape that they might come, and says they'll Apparate to Hogsmeade under the Cloak.
Returning to Hogwarts reminds the reader that this is a series about a school, and about the fact that whoever controls the school controls the public at large.