NCIS season 21 spoilers follow.

NCIS star Brian Dietzen has explained how David McCallum’s death radically changed season 21.

McCallum passed away at age 90 last year while planning for the new season was still underway. As a result, producers decided to create a special episode to write out his character, Ducky, and pay tribute to his impact on the show.

The production team turned to McCallum’s long-time co-star, Brian Dietzen — who plays chief medical examiner Dr. Jimmy Palmer — to co-write the episode “The Stories We Leave Behind” with Scott Williams.

In this emotional episode, the NCIS team comes to terms with Ducky’s death by working on one of his unsolved cases, involving a woman’s father who was dishonorably discharged from the Marines.

While Brian has written for the show in the past, he explained to TVLine that he was not planning on writing for the abbreviated season 21 until McCallum’s untimely death.

“Before David passed, we had that work stoppage with the WGA and SAG strikes, so I had said to Steven Binder and David North, our showrunners, ‘I don’t need to write this year,’ because we only have 10 episodes,” he explained.

“I didn’t want to insert myself. But then when David passed away, Scott Williams, my co-writer, volunteered to do this episode and he said to me, ‘I thought it was only right that you co-write with me.’ And then everybody in the room said, ‘Yeah, that’s perfect. That’s what should happen.’ And I said, ‘Absolutely, I’d be honored to.’”

The episode will center around Jimmy’s reaction to his colleague’s untimely death, including the revelation that it was Jimmy who’d found Ducky dead.

“Since Jimmy worked so closely with Ducky for a couple decades, it made sense that a lot of the story would be retold through his eyes and his memories,” Brian said.

“And certainly, McGee had been there with him as well, but what was really important to Scott and me, and to the whole crew, was that we honor Ducky’s relationship with the larger team. Not just the current team of Parker, Knight, Torres, McGee, and Kasie, but also the teams that came before.”