Over the course of his Star Trek career, Captain James T. Kirk (William Shatner) becomes one of Starfleet's most accomplished officers. When Kirk was introduced in Star Trek: The Original Series as captain of the USS Enterprise, he was already well on his way to becoming a legend. Captain Kirk and his First Officer, Mr. Spock (Leonard Nimoy), soon became pop culture icons and their original adventures spawned a massive franchise. After the end of Kirk's five-year mission, James accepted a promotion and continued his career with a series of big-screen adventures.
https://screenrant.com/star-trek-william-shatner-paul-wesley-kirk-promotion-demotion/