After decoding a message from their Uncle Elliott while visiting Olympic National Park, The Campground Kids are ready to continue their hunt for the Lost City. Soon Ethan, Isaiah, and Sadie are headed to Joshua Tree National Park-this time without their parents!

The kids follow a trail of clues leading to the legendary hidden obsidian spear. All too soon, they discover they are not alone and must stay ahead of their shadowy opponent who is hot on their heels. With tight spaces, mysterious engravings, and a hidden cave standing in their way, they must rely on their wits and bravery like never before.

On this journey, Isaiah, Sadie, and Ethan learn that adventure isn't just about finding treasure; it's also about friendship and teamwork.