CBS has announced this week that it is cutting the current season of NCIS short due to the coronavirus outbreak, meaning that it will end after 20 episodes rather than the usual 24. This means that there are only three episodes left of the season, with NCIS Season 17, Episode 18 the first of these. However, CBS viewers will have to wait to watch that episode, titled "Schooled," as it is not set to air this week.
Why is NCIS Season 17, Episode 18 not on this week?
Though NCIS production has been shut down due to coronavirus, this is not the reason that the latest episode is not airing this week.
In normal circumstances, network stations often have to give their shows weeks off from airing to spread episodes across the entire TV season, which runs for between 35 and weeks while most shows have at most 24 episodes. Although part of this problem is solved by giving the series long hiatuses during the holidays, most shows still have to take breaks during the September to May season.
Not including its winter break, this is the fifth time there has been more than a week-long gap between episodes of NCIS Season 17, with the show taking breaks in late October, early December, early February and late February/early March.
Instead of airing NCIS Season 17, Episode 18 this week, CBS will instead air "Into the Light," (Season 17, Episode 2), the action-packed second part of the arc that saw Ziva David (played by Cote de Pablo) return to the show.

When will NCIS Season 17, Episode 18 air?
"Schooled" is scheduled for broadcast on CBS on Tuesday, March 24, when it will air in its usual timeslot of 8 p.m. ET. The following week (Tuesday, March 31) will see the penultimate episode of the season air, though the network has not revealed when the season finale will air yet.
As for fans wondering what will happen to the four episode of NCIS that have presumably been written but that have not been filmed yet, McGee actor Sean Murray took to Twitter to say that these would air as part of Season 18 if it gets renewed by CBS.
He said: "As many of you have already heard, #NCIS production has been put on hold for the time being. we will wrap our current season with 20 eps complete and plan to pick up where we left off when we begin season 18."
He followed this with another tweet saying: "For those who have brought it up - we have not been officially renewed yet, so in no way am I announcing a pickup or anything. please understand that."
NCIS Season 17, Episode 18 will air Tuesday, March 24 at 8 p.m. on CBS.