In the fourth installment of the New York Times bestselling Charlie Thorne series—which #1 New York Times bestselling author Chris Grabenstein called “a real page-burner”—Charlie searches for Isaac Newton's secret treasure across Europe and Australia. Charlie Thorne is a genius. Charlie Thorne is fearless. Charlie Thorne may have finally met her match. Charlie Thorne is used to being on the run. Ever since she was recruited by the CIA to track down Einstein’s most dangerous equation, Charlie—and former CIA agents Dante Garcia and Milana Moon—have traveled around the world to prevent history’s greatest discoveries from falling into the wrong hands. But after beating others to the secrets hidden by Einstein, Darwin, and Cleopatra, they find they are not the only ones searching for an immensely powerful discovery of Isaac Newton’s. From a chase over the rooftops of Cambridge University to scaling the Sydney Harbour Bridge to skirting a volcanic eruption, Charlie will need to think one step ahead of her opponents to follow Newton’s trail of devious clues and keep herself out of the hands of the many enemies who are hot on her tail.