Part III is named "The Vessel" and is measured not in chapters but in hours. The kids all call their parents to explain and then settle into roles within the complex ecosystem of the car. Lacey catalogues the supplies in the car, of which there are not many (including a lack of clothes for both Ben and Radar). Radar's job is "Research and Calculations"(p.244), which means he is calculating exactly how fast they need to go and how frequently they are able to stop for gas and supplies. Ben's role is "to need to pee"(p.245), which is a joke until hour three when Ben is forced to pee into two hastily-emptied bottles of Miller Lite. Excitedly, they play games, chat, and plan for their first pit stop.
In hour four, they pull into a gas station for their first pit stop. Again, they have well-defined roles and a tight schedule. Radar pumps gas, Ben heads to the bathroom and then seeks out nonperishable supplies including clothing, Lacey explains what's happening to the cashier and then goes for liquids, and Quentin gathers food. Going just over time and under $100, the four make it back into the car and speed off. Silliness ensues as Quentin has forgotten to buy food that Lacey likes, Ben has bought Radar (who is black) a Confederate Flag T-shirt, and Radar and Quentin play a game in which they imagine the lives of people around them. The car is now almost a house, with committed areas for different acts (sleeping, relaxing, eating) and a schedule in which people move about. Around hour nine fatigue gives way to twitchiness for Radar and Quentin, who have been each had multiple energy drinks. At 12:13, in hour ten, they stop again, peeing, restocking food, and getting Radar a new set of clothes. They stop at hour eleven again because Lacey needs to pee and Quentin switches back into the driver's seat even though he has already taken more turns than the others. Just when it seems as if he and Ben are going to fight over whether he really understands Margo, Quentin sees that they are speeding toward two cows standing oblivious on the road.
Quentin panics, taking his hands off the wheel and thinking about death and Margo. However, he finds that there is still a hand on the wheel - Ben's hand, steering around the cows. Somehow, with only Quentin hitting his head enough to bleed superficially, Ben has gotten the four of them and the car safely past the danger. Ben screams belatedly, Quentin cries, and Lacey assesses the situation. They hear dripping which Ben thinks is a gas leak about to cause the car to explode but is actually 210 broken bottles of beer. After deciding the car seems fine even after it "spun around like eight times"(p.271), they set back off on their way, unable to talk about anything but what a hero Ben is and how lucky they are that he saved their lives, though he protests repeatedly that in the moment he only did it to save his own.
Quentin goes to sleep in hour fifteen and for the next three hours all there is to report is "I sleep"(p.275). He wakes up at 9:42AM, just a few hours from reaching Margo. After "Hour Twenty-one"(p.278), the next section title reads simply "Agloe"(p.279). They drive through Roscoe, the closest small town, looking for a sign of the Agloe General Store. They see a barn that they think may be it, and Lacey points out a silver car - Margo's car.
Quentin enters the building and finds it full of holes and soggy wood. Ben enters and draws Quentin's attention - in a far corner, two panes of Plexiglas form a makeshift cubicle through which they see Margo sitting in a chair at a desk, writing. All four of them approach Margo and she finally looks up, her eyes not surprised but "silent"(p.281) and "dead"(p.281). She quickly angers Lacey, Ben, and Radar and they all storm off back to the car. Turning on Quentin, she yells at him for showing up without warning and when he rebuts that he couldn't have warned her because she disappeared, leaving everyone to think she was dead, she fires back that he didn't actually find her to make sure that she was okay but instead to feel like a romantic hero. After each has made several valid points about the selfishness of the other, it becomes clear that Margo really did not intend for Quentin to find her. They begin to calm down, Margo fussing about her new haircut, calling Lacey to apologize, and explaining how she's been living in the abandoned General Store. As she reveals, she has been reading and writing for most of the day and going infrequently for supplies and showers at a truck stop. She had been planning to leave for New York City that day, hence the time limit on Omnictionary.
Quentin asks Margo about her journal and she replies with a long story: as a young girl, Margo started writing a story in which she and a fictional and more heroic version of young Quentin (along with a super-powered, talking version of her dog Myrna Mountweazel) solve the case of Robert Joyner's death. A few years after writing that, she began using the black journals for planning her schemes - running away to Mississippi, the huge TPing campaign, and more - all in crosshatch so that it is comprehensible only to her. She describes her idea of a final great adventure with Quentin, to create a final "badass"(p.292) memory for the two of them and then disappear. However, she says that she was surprised that he was not such a "paper boy"(p.292), so she decided to leave the Osprey (the abandoned mini-mall) to him through a series of clues - the Guthrie poster, record sleeve, "Song of Myself," and doorjamb note.
Margo invites Quentin to travel with her, but he turns the idea down in favor of his planned next step of college. They decide to go for a walk, and on the way Margo calls and talks to her parents and then her sister Ruthie. They hold hands briefly and then find a patch of grass to lie on together, just as Quentin imagined and wished they would when back at SeaWorld. Quentin wakes up later to find Margo digging a hole; when asked, she says, "We are digging graves for Little Margo and Little Quentin and puppy Myrna Mountweazel and poor dead Robert Joyner"(p.300). Digging the hole deep together, they talk about Robert Joyner and metaphors and finally they kiss. Margo invites him to run away with her again and he declines again, so they bury the book and return to the Agloe General Store holding dirty hands.
They make plans to stay in contact and to even visit one another. Margo cries and Quentin kisses her one last time, with both their eyes open. The novel ends with them staring at one another - seeing one another.
The car game that Quentin and Radar play called "That Guy is a Gigolo" is a classic example of sonder. Sonder was a word made up by John Koenig for The Dictionary of Obscure Sorrows, a compendium of invented words with the aim to fill a hole in language. Sonder's definition, as Koenig describes it, is "the realization that each random passerby is living a life as vivid and complex as your own—populated with their own ambitions, friends, routines, worries and inherited craziness—an epic story that continues invisibly around you like an anthill sprawling deep underground"(dictionaryofobscuresorrows.com). Likewise, Quentin notes that, "There are so many people. It is easy to forget how full the world is of people, full to bursting, and each of them imaginable and consistently misimagined"(p.257). Quentin and Radar collectively realize that while the game is fun, it really reflects what they want to see, even what is most entertaining to see, rather than what's real. This effect is underscored by the fact that they are again seeing these passersby through two sets of windows - a motif discussed earlier. This realization parallels Quentin's attempts to understand Margo, though still unable to truly comprehend her humanity.
Green writes an interesting moment of false foreshadowing when Quentin sees the cows in the road and notes that they are the things "that will shortly kill me"(p.267). Perhaps underscoring a lack of ability to predict the future (similar to Quentin's inability to comprehend what will happen when he is reunited with Margo, Quentin is not killed by the run-in with the cows. The experience is the classic kind of memory one carries on from their teen years - having a quick near-death experience and yet coming out unscathed and marveling at one's own vulnerability and invulnerability.
Margo's childhood story of a more heroic young Quentin provides Quentin and the reader with an interesting swapped point of view; Quentin suddenly experiences the slant of his own perceptions focused back from Margo onto himself. In a meta-literary moment, Margo explains that she had to kill Quentin in the end of her childhood story because the other option was for them to have sex and she "wasn't really emotionally ready to write about [that] at ten"(p.290). What is meta-literary at this moment is that Green's book is also coming to a rather tidy close, only this time Margo and Quentin do get at least a semblance of their romantic ending.
Soon after this conversation, Green has Margo and Quentin physicalize the theme of "coming of age" with the literal burial of their past selves and dreams in the form of Margo's journal. They drop dirt on the journal and invoke the memories of their own childhood selves along with Margo's dog and Robert Joyner. This symbolic moment is done in hopes that they are able to close the book on their childhoods and move on into adulthood. There does seem to be maturity in the next few pages that end the book, but this moment can also be viewed cynically as in vain, since Quentin and Margo both have much more growing up to do before truly being adults, and childhood is never truly left behind.
Green ends the book with Margo and Quentin kissing in the dark with their eyes open and then simply staring at each other with their foreheads touching. Quentin's final line of narration is "Yes, I can see her almost perfectly in this cracked darkness"(p.305); but, as with the journal experience moments before, this seems to be a moment of overconfidence that demonstrates what he didn't learn from his misinterpretation of Margo's clues and his realization in the car regarding "That Guy is a Gigolo." Quentin still feels as if he can really see who Margo is, driving home his childishness and possessiveness of Margo to the last moment of the book.