be on the straight and narrow

To maintain a morally upright way of life; to only make choices that are considered morally and legally correct. Now that I have a conviction on my record, I really have to be on the straight and narrow going forward. My father was on the straight and narrow until the day he died, and I think he regretted never doing anything wild or unexpected when he had the chance. Bill got himself into a lot of trouble as a teen, but he's really been on the straight and narrow ever since.