'NCIS': Original Cast Member Returns for Season 20, Episode 21

David McCallum, the last remaining original NCIS cast member still working on the show, made a rare appearance during the penultimate episode of Season 20, which aired on CBS Monday night. McCallum, 89, has played Donald "Ducky" Mallard since the show began in 2003. The character was introduced as the medical examiner, but that role is now filled by his assistant, Brien Dietzen's Jimmy Palmer.

Since retiring as M.E. during Season 16, Ducky took the role of NCIS historian. This job came in handy for the main team in "Kompromat." The episode saw the ongoing Russian spy ring plot reach a crescendo before next week's season finale. A National Archives Museum guard was found dead among confidential files, around the same time Sen. Constance Miller (Brigid Brannagh) accessed the archives. Miller insisted she had nothing to do with it since she was spending time at Parker's (Gary Cole) place.

During the investigation, the State Department agreed to a swap with Russia, sending Nate Billings (Sonny Valicenti), Evelyn Shaw (Juliette Goglia), and identity thieves John and Reece Watts in exchange for other international prisoners. The Wattses are even already on a plane to Russia. Knight (Katrina Law) and Torres (Wilmer Valderrama) then head off to the USS Cortland to meet Billings. Billings was really happy to go back to Russia, at least until he heard that his handler Yuri was going to be the one meeting him there. He is so spooked by Yuri that he takes his own life in his cell.

Who is this Yuri guy? Parker learned that Yuri stole a file on a 1985 Russian defector named Ilya Sokolov. Since that file was never digitized, there's only one person who can solve this mystery... Ducky! He Zooms in from Edinburgh, where he was on a stop for his lecture tour. He remembered Sokolov was a brilliant polymath who became a professor in the U.S.

The team headed to Sokolov's home, where he was tortured to death by Yuri (Themo Melikidze). The Russian was surprisingly happy to go to prison since he thought there was nothing left for him to do. We later learn that he may have gotten information about a weapon Sokolov was working on! If he sent that information to the Russians, the project could already be in the works. Fans will have to tune into the finale "Black Sky" on May 22 at 9 p.m. ET on CBS to see how this all ends.

McCallum, who celebrates his 90th birthday in September, rose to fame as Illya Kuryakin on The Man from U.N.C.L.E. in the 1960s. He is one of the last surviving actors who starred in The Great Escape. In 2003, McCallum was introduced to a new audience when he joined NCIS. He has had a limited NCIS schedule since 2017. After Mark Harmon left in the fall of 2021, McCallum became the last remaining star from the Season 1 main cast still on the series.