A piece of lumber projecting from the side of a freight car picks up the mail sack hanging from the crane at Bell Station. Two hobos find the sack and rifle the contents. Morrison, postmaster at Bell, has been systematically robbing the mails and benefits by the mail sack's disappearance. Mary Gates, daughter of an engineer, supplies the Post Office inspectors with the first clue. The detectives blunder, however, and arrest Billy, Mary's sweetheart. Chance leads them on the trail of the tramps. The hobos are arrested after a desperate battle. Later, as the result of Mary's clever work, Morrison is brought to bay, while Billy wins his freedom.