Arthur's Mystery Envelope is the second part of the tenth episode of Arthur Season 2. Arthur's Mystery Envelope aired on December 8, 1997. It aired with the episode "D.W. Goes to Washington".
Mr. Haney gives an envelope to Arthur to give to his mom. Arthur is worried about the contents of the envelope, thinking that it's a negative report about him, so he comes up with a way of giving it to his mom without her opening it. As it turns out, the letter was paperwork for his mother that had nothing to do with him and he is relieved that he had nothing to worry about.