Peter and Langdon wrestle over how to save Katherine.

Langdon, Katherine and the CIA spring a trap for Mal'akh.

Katherine and Langdon persuade Isabel to protect herself.

Mal'akh sheds light on Zach's final days in prison.

Mal'akh pressures Langdon and Peter to decipher the clues.

Langdon reunites with Katherine and teams up with Sato.

Langdon follows the next clue; Katherine teams up with Sato.

Chased by the CIA, Robert and Katherine seek Nuñez's help.

Robert and Katherine try to decipher the most recent clue.

Robert must solve a mystery to save his kidnapped mentor.