When Jacob Portman, our narrator, is a wee little child, he wants to be an explorer. Of course he does—his middle name is Magellan (so he either wants to be an explorer or an early '90s dragon-shaped puppet), as we learn when we read the inscription in the book his grandfather leaves him.
His grandpa really encourages his imagination, helping him plot "imaginary expeditions with trails of red pushpins and telling [him] about the fantastic places [he] would discover one day" (Prologue.3). But sadly, Jacob stops believing in his grandfather's stories and starts getting increasingly discouraged and cynical, more so than most teen boys, even. Together, Jacob and his dad write off Grandpa Abe's rants about monsters and magical realms as a sign of old age.
Despite outgrowing his gramps, and even though Jacob has it pretty good—his mother's side of the family owns a drugstore empire—he is lost in life. He acts like an entitled rich kid, trying to get fired from his job, and saying, "No matter how many displays I knocked over or customers I short-changed, one day I was going to inherit a sizable chunk of the company, and they were not" (1.19). Well then, fancy pants.
All this changes, though, when Jacob's grandfather dies in his arms. While this is the kind of thing that would change anyone, the way in which Grandpa is killed—by a monster that only Jacob can see—changes things more than Jacob ever could have imagined.
Jacob suffers from nightmares after his grandfather dies, and his psychiatrist, Dr. Golan (more on him elsewhere in this section), suggests deciphering Grandpa's last words as a way to help Jacob cope. That youthful yearning for adventure crops back up inside Jacob, and he's kind of excited and rejuvenated when he finds out that his "grandfather's last words began to make a strange kind of sense" (3.111). It's the beginning of an adventure.
And what an adventure it is. Jacob ends up discovering that what his grandfather said was true: Peculiar children with superpowers live in a time loop on an island off the coast of Wales. Plus, Jacob is peculiar, too, in the best possible way, as he can see monsters like his grandfather did.
This revelation gives our main man a new purpose in life, and he decides to leave his boring ol' mundane existence behind and stay in the world of the peculiars, partially because it's what he's always wanted, and also because he's kind of in love with another peculiar child, Emma Bloom (be sure to check out her page in this section). Fun fact: Emma was also Grandpa's girlfriend. Okay, by fun we kind of meant gross, but whatever.
So in order for Jacob to grow up, then, he has to return to what he believed in as a child. In this way, it's like he's gone full circle, just like Magellan circling the world. As this book ends, we're left waiting to see what comes of Jacob's commitment to a life amongst the peculiar.