NCIS: Los Angeles season 12 episode 17 serves as the penultimate of the current installment, which comes with the title "Through the Looking Glass." Some of the stories explored earlier this season will resurface, with David Kessler (Frank Military) coming back to bring more chaos.

Kessler's return in NCIS: Los Angeles season 12 episode 17 is going to be an eventful one for Marty Deeks (Eric Christian Olsen) and Kensi Blye (Daniela Ruah). The latter is, particularly, in danger given that this man has been obsessed with her for quite a long time already.

Kessler's whereabouts have been a mystery through the early run of the current season. Kensi's safety was a major concern after he suddenly disappeared. Clearly, he is hiding somewhere and planning to make an unexpected attack at some point.

Now that he's coming back in NCIS: Los Angeles season 12 episode 17, this will be a huge problem for Deeks and Kensi. They know how dangerous Kessler could be, and they won't feel safe as long as this man is out there on the street.

As the synopsis notes, Kensi received a postcard from Kessler, the "sociopath who is obsessed with her." In the promo, he sent the said postcard to the team and it is addressed to her, alarming everyone -- most especially her husband, Deeks.

It looks like Military's character is going to be a major adversary moving forward with the season finale. Kessler may cause chaos, which could interrupt the team's latest case. His presence could also build up more stories leading to the next installment.

As for what else is coming in NCIS: Los Angeles season 12 episode 17, a Naval Intelligence Officer has been tortured and murdered. This prompted the team to work with Joelle (Elizabeth Boush), who told them that CIA agents were also killed in the same way.

Since this is the penultimate, there could be more unexpected twists and turns moving forward. For Kessler, he might be the "end-of-season villain" since most of the characters were seemingly afraid of him. Joelle's storyline, on the other hand, may also continue until the final episode.

The good news is that Hetty Lange (Linda Hunt) is coming back before the current installment officially wraps up. She could help the team in handling the case they're dealing with at the moment, as well as finding a way to bring Kessler down for good.

NCIS: Los Angeles season 12 episode 17 airs on Sunday, May 16, on CBS.