The Upper Case: Trouble in Capital City (Private I)

Just when Private I thinks all is calm--now that he's cracked the case of 7 Ate 9--Question Mark storms into the office. Mark is worried. All the uppercase letters are M-I-S-S-I-N-G! But that's absurd. This is CAPITAL City! Private I is the last letter standing. Will he solve his BIGGEST mystery yet, the UPPER CASE, before it's too late?!