Mark Harmon left NCIS in season 19, changing the dynamic of the show. In storyline terms, Gibbs left due to his involvement in a case where he broke the law and intended to serve time for his crime. However, Agent Parker saw the good Gibbs had done and disobeyed orders to arrest him, resulting in Gibbs deciding to stay. In real life, Harmon was ready to move on and negotiated a limited appearance in season 19 to ensure the show's renewal. The door remains open for Gibbs to return in the future.