On the last episode of "NCIS: Los Angeles," Kensi and Deeks went undercover in a rehab facility to crack a murder case.

In "Recovery," a person was murdered in the pool of the Tranquility Villa rehab center. The victim was Navy Intelligence Officer Gary Leonida, who checked himself in with an addiction to oxycodone. The toxicology report shows that a drug used in interrogations was also found in his system. The NCIS team suspects that someone wanted to know information to whom he had access.

The team goes through multiple suspects, including the owner of the rehab center and Gary's "sober buddy," who turned out not to be very sober. However, Kensi (Daniela Ruah) and Deeks (Eric Christian Olsen) find that they are innocent.

Kensi and Deeks go undercover at the rehab center as a nutritionist and a patient with a sex addiction, respectively, in an effort to crack the case. Deeks is able to name a woman who was chased from Gary's room an hour before his death. However, two men with guns track her down before they get to her. Once the team rescues her, she says that Phil, the new meditation teacher, was seen coming out of Gary's room. Phil is really Robert Hayes, part of the Homeland Defense League. Eric (Barrett Foa) finds out that Hayes used the intel he got from Gary to gain control over a US predator drone in Afghanistan.

The team tracks down Hayes, but find out there is someone else targeting the village that was the home of the Pakistani ambassador to the U.S. They scour his motel room and find a nutrition bar that Kensi knows only a certain person ate at the rehab center. They get to her just in time to disarm the missiles and arrest her.

Then after the case, Deeks finally reveals his true feelings to Kensi over a romantic dinner.

On future episodes, Kensi and Deeks will face big obstacles. Executive producer Shane Brennan spoke to TVLine about the characters' relationship.

"Kensi and Deeks face their biggest challenge, both professionally and personally in coming episodes," Brennan said. "The emotional ripple that started with their unexpected kiss at the end of last season is about to turn into a tsunami that doesn't just impact them, but the whole team. As a result, one of them will be reassigned."

It's also been revealed that "Gossip Girl" star Michelle Trachtenberg will guest-star in an upcoming holiday episode of the drama. Trachtenberg will appear in the Dec. 17 episode as Lily, a smart, charming environmentalist/chef. Lily will endure a scary home invasion, but will do her best to put on a brave face.

"It's a very intense episode which pairs Michelle with Callen and Sam (played by Chris O'Donnell and LL Cool J), in what is probably one of our biggest action shows so far this season," Brennan said.

On the next episode, "The Frozen Lake," "Callen and Sam are going to enlist the help of a Gurkha, an elite Nepali soldier, specializing in knife fighting, to retrieve a missing thumb drive. Meanwhile, Kensi and Deeks are going to struggle to find a balance in their partnership. Guest stars will include: Ernie Reyes Jr. as Jemadar Thapa."

Watch the promo for "The Frozen Lake," which airs Tuesday at 9/8c on CBS.