Geronimo investigates to see if the famous painting Mona Mousa holds a secret code.

Unable to return to their normal lives despite the end of the Clue hunt, new Madrigal leaders Amy and Dan anxiously prepare for a retaliatory strike by the Vespers, who kidnap a number of Cahill family members and demand that Amy and Dan steal a famous painting.

Akiko, Mae, and Josie, also called the Infinity Trinity, spring into action after learning that a spy is betraying secrets to the Japanese military--and that Akiko's mother may be involved.