The hunt for the missing spy

"Cody, Queen, Luke, and M.E. are the Code Bustersclever clue hunters with a passion for puzzles. They can't wait for their school trip to Washington, D.C., where they'll get to visit the International Spy Museum. But it seems as if someone is spying on themor at least leaving coded messages that even the Code Busters can't crack. And once they arrive at the museum, they notice they're being followed by a mysterious stranger. Then a classmate goes missing. Could the unknown spy be responsible? It's up to the Code Busters to find out!" --Publisher's description.