To Quentin, Margo is the most 'fantastically gorgeous creature that God had ever created.' He remembers playing together and biking to Jefferson Park at nine years old. At the park one day, they discover a dead man slumped against the trunk of an oak tree. They later find out that the deceased man was named Robert Joyner and that he had committed suicide due to the grief and stress of his divorce.

Straightaway, John Green highlights the difference between the two children's personalities and backgrounds from the way they respond to such a disturbing find. Quentin's parents are both therapists, and Quentin has a matter-of-fact chat about life and death with his mother. On the other hand, Margo has a dysfunctional relationship with her parents; she chooses to instead investigate everything she can about the man by chatting up her fellow neighbor, Mrs. Feldman, and Robert Joyner's next door neighbor, Juanita Alvarez. Margo even speaks to some police officers she sees at Joyner's apartment building.

Both Margo and Quentin grow apart as they mature through their school careers. It is only when Margo approaches Quentin about wrecking vengeance on people who have hurt her in life that the story starts to take on the main conflict of the novel. Basically, Quentin and Margo have to come to terms with how they want to relate to each other and how they want to resolve differences between the two of them.

In a nutshell, both eventually decide to go their separate ways. The discovery of the dead man becomes the catalyst which brings about their gradual estrangement. Both harbor preconceived and prejudicial views about the other but do not realize this until later in the novel. In the meantime, Margo secretly wants Quentin to be her hero, while Quentin privately thinks that Margo is already the perfect girl at nine.

Toward the end of the novel, Quentin learns that finding the dead man prompted Margo to write a story; in it, Quentin was the fearless hero who was willing to die to protect her. Both of them worked together to investigate the death of Robert Joyner. Margo confesses that, in her story, Robert Joyner's death was not a suicide but a murder. When both of them confront the killer, the killer takes a shot at them, but Quentin jumps in front of Margo at the last second and dies in her stead. Margo's anecdote plainly illustrates the psychological frame of mind she was in at the time: she desperately needed Quentin to be the hero who would rescue her from a marginalized and lonely existence in school and at home.

Eventually, she has to come to terms with the fact that both of them are two different people with different goals in life. When Quentin offers for Margo to move in with his family after graduation, she turns him down.

It's not just them...I'd get sucked right back in... and I'd never get out. It's not just the gossip and the parties...but the whole allure of a life rightly lived- college and job and husband and babies...

Quentin thinks that he may like to have all the above one day and questions Margo about her plans for the future, to which she replies that 'forever is composed of nows.' Both Quentin and Margo eventually come to the realization that they cannot live their lives through each other; both have to carve out their path in life individually.