Hazel Lancaster and Gus Waters met at a support group for people who suffer or have suffered from cancer. Imminently they has a connection and started to hangout constantly.
While hanging out Hazel talks a lot about her favorite book and the author. Gus decided to use his wish he was "granted" when he had cancer to go visit the author of the book in Amsterdam with Hazel because she used her wish already.
Soon Gus and Hazel are in Amsterdam enjoying eachother and Gus confesses his love.
The next day in Amsterdam, Hazel and Gus go to see their favorite author to a book that inspired them both. The author ended up being a jerk, and Gus and Hazel left, but that didn't mean they couldn't have fun in Amsterdam. They spent time together to recover from the disappointment of the author, and while together Gus tells Hazel that his cancer is back.
When Gus and Hazel get back from their amazing trip in Amsterdam, Gus's cancer takes full effect and he gets sicker. Though he is not dead, his family and Hazel have started to prepare for his funeral because of his condition.
Just days later Gus died from the cancer. Hazel spoke at his funeral and told of their story and her love for him.