The straight and narrow
Definition 1 of 1
Pro Tip 1/3Metaphorical Path
The phrase uses 'straight and narrow' as a metaphor for a direct and honest life course. 
After his release from jail, he promised to stay on the straight and narrow.
Pro Tip 2/3Behavioral Change
It often implies a positive change in someone's behavior or choices. 
He used to be quite the troublemaker, but he's been on the straight and narrow for years now.
Pro Tip 3/3Figurative Use
The phrase doesn't mean a person is literally walking straight, but acting in a morally correct way. 
Despite peer pressure, she chose the straight and narrow.