Mark Harmon is stepping back into the role of Leroy Jethro Gibbs after his exit from NCIS. CBS announced on Tuesday, October 28, that Harmon, 74, will make a special guest appearance during the first crossover event between NCIS and NCIS: Origins. The two shows intersect on Tuesday, November 11, with younger Gibbs (Austin Stowell) and the team investigating the small-town death of a naval officer in the '90s.