Hazel, Augustus, and Hazel's mom all fly to Amsterdam to meet Peter van Houten to find out the ending of the book. There is an organization that helps cancer kids dreams come true and they pay for the entire trip.
The climax of the story is when Augustus dies when his cancer spreads
Isaac told Hazel that Augustus had said that he was writing something for her a few weeks before he died. She couldn't find it and she realized that it was the letter that Peter Van Houten had from Augustus. Hazel e-mails Lidewij and asks her if she can go and get the letter for her and she sends it to Hazel over e-mail after getting it from Peter's house. The letter is asking Peter Van Houten to write a eulogy for Hazel.