The Title of the Season 18 Finale of 'NCIS' Hints [SPOILER] Is Leaving

Spoiler Alert: This article contains spoilers for Season 18 of NCIS.

Part of preparing ourselves for the Season 18 finale of NCIS means Googling everything we can about the episode and absorbing as much information ahead of time. And obsessing, theorizing, and setting up a countdown on our phones, obviously. One major clue as to what the episode is going to be about is in the title of the season finale: "Rule 91." What exactly does it mean? 

"Rule 91" references Leroy Jethro Gibbs' (Mark Harmon) boat, which is named Rule 91. This is something we most recently learned. It also references Gibbs' rules in general, which are all essentially the rules he requires his team to follow. There are 69 in total, but not all of them have been revealed.

Since there are only 69 (that we know of), Rule 91 seems to be specifically tied to the Season 18 finale episode, and it could very well be about Mark Harmon's exit from the show, which has been hinted to.

Above and below are photos of Rule 91 (the boat), which were recently released by CBS. We already know that Gibbs builds sailboats — it's a major hobby of his. Throughout all 18 seasons, Gibbs has built six boats, including this latest one, and each one always has a significant tie to his story. He also tends to name boats after someone close to him (which isn't super unusual for boaters, but still). 

Will Gibbs use this last boat to sail away into the sunset? Mark actually thinks so. Back in 2010, he said, "From the beginning here, I always thought that part of his manner in dealing with his team is to prepare them for a time when he's not here. So, I could see a time certainly in the future where they take the job over and they move on, and Gibbs gets on his boat and sails away." Did he really hint at Gibbs' exit over 10 years ago?

According to the promo for the Season 18 finale, we might have finally learned what it all means. And it's not great news. When Bishop (Emily Wickersham) asks Gibbs what Rule 91 is, the promo cuts to him saying, "When you decide to walk away, don't look back." Did Gibbs decide to walk away from NCIS forever, or is he giving advice to Bishop that it's time for her to walk away?

The only certain clue we get about the episode is the synopsis which comes from CBS: "While pursuing a dangerous arms dealer, the team is shocked when Bishop is implicated in an old NSA leak. Also, Gibbs and Marcie (Pam Dawber) realize that the killer they’ve been tracking may be onto them, on the 18th season finale of NCIS."

And as far as Bishop goes, Distractify has several thoughts about whether or not Bishop is making an exit, but ultimately, with her relationship with Torres (Wilmer Valderrama) finally heating up, it seems unlikely that she'll be the one to go. However, it's also worth noting that Gibbs will definitely be back next season, but what remains unknown is in what capacity. Will he be a guest star who pops in to help the team every now and again, or will he still be a series regular?

Catch the Season 18 finale on May 25 at 8 p.m. EST on CBS.