The Code

"Another house, another town, another year without Mom. For 12-year-old twins Andrew and Evie, moving is a way of life. Their dad is in the military and they've never stayed in one place for long. But relocation has gotten harder since their mother disappeared on the twins' eleventh birthday, leaving only a cryptic note.
At first their new town seems like all the others. Then Andrew and Evie receive a mysterious package filled with random items. They have been chosen to perform an important and dangerous task. Everything they encounter is more complicated than it appears and no one's identify can be relied upon. They have only each other ... and Spy X."