'NCIS: The 19th Season' is out on DVD on Aug. 16. After 18 years, Mark Harmon retires his lead role as Special Agent Leroy Jethro Gibbs. The five-disc collection includes all 21 episodes, as well as special features, behind-the-scenes moments and more. The 19th season kicks off with a search for Gibbs (Harmon) after wreckage from his boat is discovered and he is nowhere to be found. After finding out about Gibb’s secret mission to track down a dangerous serial killer, Agent Jessica Knight (Katrina Law) joins Special Agents Timothy McGee (Sean Murray) and Nickolas Torres (Wilmer Valderrama) to help locate him. From murder and espionage to terrorism and stolen submarines, these special agents investigate all crimes with Navy or Marine Corps ties.