For starters. The first two episodes of NCIS season 18 were intended for the previous season, which ended prematurely due to the COVID-19 crisis. The season 17 finale also would have been the 400th episode of the series.

So, ahead of the new season, there are big plans for episode 400. Speaking to TV Insider, show executive producer Frank Cardea, 73, revealed the milestone hour-long episode will focus on special agent "Leroy Jethro Gibbs" (Mark Harmon) and his long-time friend, the coroner "Dr. Donald 'Ducky' Mallard" (David McCallum).

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"It's a very special episode," Cardea said of NCIS 400. "It's about Gibbs and Ducky and how they met. A case brings them back to 20 years earlier."

With the episode exploring "Gibbs'" past, Mark Harmon's son Sean Harmon, 32, returns to play the younger version of the NCIS agent. "The goal for this episode was to write a love letter to the longtime fans," the NCIS executive producer added.

"Gibbs" and "Ducky" both are an integral part of the NCIS cast.

NCIS 400th episode: Release date?

Filming of NCIS season 18 resumed in early September. The cast and crew have already completed the production of the upcoming 400th episode, as series star Wilmer Valderrama, 40, announced on Sept. 18 on social media.

NCIS season 18 will premiere on CBS sometime in November 2020, in the United States. The 400th episode is to follow immediately after the season premiere and will air as the second episode of the new season.

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