A boy describes his life with his family in San Francisco. He loves hunting for fossils and creating cities from old cardboard.

The boy and his father share a bond for their old blue bus, named Neptune. She's rusty, rickety, and her motor doesn't always start. Still, they both treasure her, even while others do not.

One day when the family prepares to head out on a fossil hunt, Neptune's engine once again fails. She proves her worth by reminding everyone to slow down and appreciate one another.

Neptune tells the story of an observant boy, his patient father, and their deep love for an old bus with the power to change an entire neighborhood.