Doublecross is the fourth series in The 39 Clues universe. It is preceded by the Unstoppable Arc and it is unknown if there will be a series after Doublecross.
The Doublecross series focuses on a new villain, only known as The Outcast. They are trying to recreate world disasters and rekindle the sparks of branch rivalry, and it's up to the Cahill family, now lead by Ian Kabra to stop them.
Series 4 of the 39 Clues was first announced during Worlds Collide in April 2014, while the first information about it was revealed on the 8th of September. It will be a four book series.
The Outcast was removed from the Cahill Family. He was waiting, recruiting, planning his revenge. Now he strikes, by recreating four disasters. The Cahills have only days to stop him.
It was released on the 24th of February, 2015.
Released on July 28, 2015.
Book 3: Mission Hurricane by Jenny Goebel
Released on January 26, 2016
Book 4: Mission Atomic by Sarwat Chadda
Released on June 28, 2016