Doublecross is the 39 Clues series that is after the Unstoppable series. It is the fourth arc in The 39 Clues, and there are four books. The authors of the series are Jude Watson, C. Alexander London, Jenny Goebel, and Sarwat Chadda. The series logo is yellow, after the Ekaterina Branch. One card from each book will form The Outcast Master Plan Puzzle