S3·E2 Lester, the Skipper

With his letters home, Gruber has led his girlfriend to believe that he's a Lieutenant Commander, and, when the girl pays a surprise visit to Taratupa, McHale reluctantly agrees to help him save face, by allowing him to act out the part. The charade almost gets them into big trouble, but winds up working out for the good, when it helps foil Binghamton's plan to have McHale assigned to a British Liaison's position.