Marnie, with the Shoebox Zoo in tow, flies to Denver for the summer in order to search for The Book of Forbidden Knowledge. After being picked up by her grandparents, she meets up with her best friend Kyle Stone and plans to show the shoebox animals to him. However, a mysterious figure has stolen the bag in which they were contained, on top of seemingly following her around wherever she goes.

Kyle finds Marnie’s bag on his way to visit her for breakfast. With the proper bag now on hand, Marnie introduces Kyle to the shoebox animals and tells him about the Quest. Later, they’re taken a TV studio where they meet Aurora Dexter, a medium who aspires to become a TV star.

After settling in a hideaway that’s owned by the Campbells, Marnie and Kyle go to investigate the first clue, a snake rune, at the nearby Snake Creek Trail. After an argument causes the two to split up, Marnie has an encounter with the ghost of Toledo, the shapeshifter.

Marnie and Kyle go to a Native American festival called “The Pow Wow”. While there, Marnie comes face to face with the mysterious figure who’s been following her around since she arrived back in Denver.

Marnie and Kyle go bike riding in the woods in order to investigate the next clue, a bear claw rune, but things go awry when the shoebox animals fall out of Marnie’s backpack and are spotted by a wild bear. Meanwhile, Toledo puts his plan to bring about the second prophecy into motion.

It’s Kyle’s 12th birthday, and so Marnie attends his birthday party. Toledo, now possessing Aurora’s body, spies on Marnie at the party and encourages her to find the book, attempting to allure her with the prospect of unlimited power.

Marnie and Kyle go to watch a rodeo. While there, Kyle confides in Marnie regarding his self-doubts as well as his reluctance to keep going with the quest, which leads to an argument between the two.

Marnie goes to Camp Healing Bow along with Kyle and some of her former classmates. Following their arrival, they partake in several fun activities, but Marnie suddenly starts exhibiting hostility towards everyone around her and no one seems to know why.

The stay at Camp Healing Bow continues. Marnie’s odd behavior seems to worsen, which puts a strain on her friendship with Kyle. Despite this, the two agree to investigate the next clue, an eagle rune, together. Meanwhile, Marnie’s father arrives in Denver to visit her, but things take an unexpected turn when Aurora picks him up at the airport.

It’s the last day of camp, and Marnie’s father comes to visit her. However, happiness quickly turns to anger when Marnie sees that he’s brought Aurora along with him, and that they’ve seemingly fallen in love with one another.

The Quest’s conclusion draws near, with only one clue remaining unsolved, the wolf rune. Marnie, now fully consumed by her anguish and grief as a result of the book’s corruption, is desperate to take the book so she could revive her deceased mother, and uses any means necessary in order to reach it.

All hope seems lost as the book has fallen into Toledo’s hands. However, upon receiving some encouraging words from a fallen ally, Marnie comes to the realization that the book must be destroyed in order to save the world, and so she ties up some loose ends in preparation for the impending final battle.