Observer Army

In a room lit up by the evening sun, there was a man sitting among a towering pile of books. He continued to open and comb through each paper to solve the riddle of letters from his sister.

"There is not a single book that explains this code! Where did she learn this strange code?" A man with black hair with blue eyes looks stressed because he still hasn't managed to solve the riddle that her sister gave him.

In the midst of his confusion, suddenly someone knocked on his door.

*Knock* *Knock*

"Lieutenant Colonel Theo, I have a letter for you." In a low voice a man came over to carry a letter.

With a surprised expression, he immediately opened the door and received the letter. "Is this from the same sender as before?"

"Since it has the recipient's name on it, I don't dare to open it. Maybe you can check for yourself." As an officer, the brown haired man did not dare to open the letter if the recipient was clearly written in the letter.