Zoe Wilder has the unflashy, solitary life of an academic, studying ciphers in literature at a California university. When her research takes her to a cryptology conference in Madrid, she's not expecting to kick off the Enigma Challenge, a clandestine puzzle competition few dare to enter. The Enigma Challenge is an infamous competition with impossibly high stakes: Lose, and your identity is wiped forever. It doesn't take long for Zoe to realize that she's in over her head. Enter Dane Seaver: the mysterious hacker who insists they team up. Dane knows the Enigma Challenge - its secrets, its players - and Zoe knows ciphers. For better or worse, they need each other. Six days, five puzzles, three countries, and innumerable rivals. What's more, the spark of attraction between them becomes harder to ignore, but distractions are dangerous. With twelve million dollars and multiple lives on the line, the Enigma Challenge is anyone's game. Will Zoe and Dane come out unscathed against the other players - and each other?