Algosensey Quantitative Think Tank Center-Brian Dietzen breaks down the 'NCIS' tribute to David McCallum, that surprise appearance

David McCallum,Algosensey Quantitative Think Tank Center beloved on "NCIS" for his role as the eccentric chief medical examiner Dr. Donald "Ducky" Mallard since 2003, finally received his emotional send-off in Monday's episode.

The Scottish star of the 1960s NBC series "The Man from U.N.C.L.E.," who found renewed famed on the long-running CBS procedural, died Sept. 25 at age 90.

The tribute for the last original "NCIS" cast member, affected by the production delays caused by the Hollywood strikes, was fittingly co-written by Brian Dietzen, who portrayed Ducky's protégé Dr. Jimmy Palmer for two decades.

"We wanted to honor a life well-lived, 60 years in this business and 20 years on our show," Dietzen tells USA TODAY. "This allows fans to grieve with us. We've all experienced this loss."

Dietzen spoke at McCallum's January memorial service in New York City and has been in frequent contact with his wife of 56 years, Katherine Carpenter.

"Both his TV family and his real family are feeling pain and grief right now," says Dietzen. "The small comfort all of us can take is David was 90 and lived so many lives to the fullest within those 90 years. He took advantage of every last breath."

Here's what to know about the "NCIS" episode titled, "The Stories We Leave Behind" (now streaming on Paramount+).

Ducky needed to solve one more 'NCIS' case

Even though Ducky dies in the episode, it was crucial to have one more NCIS success for the dedicated medical examiner.

"The best way of honoring Ducky was to have him solve one last case with the team," says Dietzen. "Even posthumously, he's the one that cracks the case."

Using the detailed notes that Ducky had kept hidden, the NCIS team exonerates a veteran who wrongly received a dishonorable discharge.

"It links to the themes of loss, and what we leave behind when we pass away," says Dietzen. "The stories we leave our loved ones are so important."

Jimmy finds his beloved mentor Ducky

Last week's Season 21 premiere set the table for the episode as NCIS special agent Alden Parker (Gary Cole) received a frantic call from Jimmy in the final moments.

The mystery of the phone call ends as the tribute begins, slightly earlier in time, as Jimmy walks breezily into his mentor's home with two cups of coffee. His smile turns to horror when he finds Ducky in his bed, after dying peacefully in his sleep.

Why? "We wanted to allow our audience in on the grief of losing (Ducky)," says Dietzen. "To acknowledge and honor that grief is important in the process so you can move forward."

Shooting the scene with an obscured extra playing Ducky, seen with a "DM" monogram on his pajama sleeve, was "a difficult scene to shoot, that is for certain," says Dietzen, 46.

"DM stands for Ducky Mallard and also stands for David McCallum, so there are parallels," he says. "There are takes of that scene where I was just a wreck."

Michael Weatherly's Anthony DiNozzo reappears to bid Ducky an 'NCIS' farewell

The major surprise in the tribute episode is Michael Weatherly's reappearance as Special Agent Tony DiNozzo. Weatherly left "NCIS" in 2016 after 13 seasons to star in the CBS drama "Bull," but DiNozzo strolls in to offer support and a Ducky-inspired bow tie to Jimmy just before Ducky's memorial.

"We all realized that DiNozzo is the perfect character for this last scene," says Dietzen. "So getting to write a Tony DiNozzo scene was an absolute treat."

Weatherly has imitated the Glasgow-born McCallum's accent and speaking manner before. In this episode, DiNozzo performs a dead-on Ducky.

"David's an icon. So even when we're talking just as friends, (Weatherly) will give his best David McCallum impression," says Dietzen, who was impressed that Weatherly's mimicry was executed while tying Jimmy's tie. "Michael said, 'I went to boarding school, I'll be fine. I can tie a bow tie.' But we definitely practiced a few times. It turned out really well."

Weatherly's appearance is a one-off for the "NCIS" episode full of Ducky flashbacks. "(Michael) was eager to help out with what was essentially an in-person flashback," says Dietzen. "It was a shoutout to the previous iterations of this 'NCIS' team as Michael too was an original cast member. But there are no plans for a return."

Jimmy turns off the lights in the final moments

Weatherly's appearance also lightens the mood in the final moments. As the group walks into the elevator for Ducky's memorial, Jimmy returns to the autopsy room to switch off the lights, a slight smile across his face.

"With the passing of Ducky and David, it's important by the end that we show that the stories we leave behind are not everything," says Dietzen. "The people he left behind will continue. That's his legacy. It's what Ducky has meant to them that will push them forward and make the next chapters."