Times bestselling Charlie Thorne and the Last Equation--which #1 New York Times bestselling author Chris Grabenstein called "a real page-burner"--Charlie searches for Charles Darwin's hidden treasure in South America.

Charlie Thorne is a genius. Charlie Thorne is a fugitive. Charlie Thorne isn't even thirteen.

After saving the world, Charlie is ready to take it easy in the Galapagos Islands. That is, until she's approached by the mysterious Esmeralda Castle, who has a code she knows only Charlie can decipher. In 1835, Charles Darwin diverted his ship's journey so he could spend ten months in South America on a secret solo expedition. When he returned, he carried a treasure that inspired both awe and terror in his crew. Afterward, it vanished, never to be seen again...

But Darwin left a trail of clues behind for those brave and clever enough to search for it. Enter Charlie Thorne. In a daring adventure that takes her across South America, Charlie must crack Darwin's 200-year-old clues to track down his mysterious discovery--and stay ahead of the formidable lineup of enemies who are hot on her tail.

When an ancient hidden treasure is at stake, people will do anything to find it first. Charlie may be a genius, but is she smart enough to know who she can trust?