The "NCIS" team spent the days leading up to the holidays figuring out what was causing a mysterious epidemic. 

In the episode, a bunch of children in military families are infected with a mysterious respiratory illness. 

After seeing the sick children, Palmer (Brian Dietzen) begins to doubt his previously eager desire to adopt a child. His doubts could not come at a more inconvenient time, as he and his wife, Breena (Michelle Pierce), are soon going to meet a pregnant woman who is interested in giving them her child once it is born. However, Palmer eventually comes around with the support of Gibbs (Mark Harmon and the rest of the team. 

It turns out that a man who had been playing Santa at military Christmas parties was in part responsible for the outbreak, which was caused by a spore from Africa that causes strep pneumonia. The spores were stolen by a disgruntled employee, and it ended up on his Santa suit. In the end, it is Abby's lab friend Carol who cracks the case. 

Meanwhile, Vance (Rocky Carroll) is avoiding interacting with his late wife's estranged father (Ben Vereen) throughout most of the episode. He showes up out of blue to try to connect with his grandchildren, but Vance wants nothing to do with him because he knew that he had abandoned his wife's family years ago. Vance therefore denied his requests to see his children. 

But when Vance overhears Abby's (Pauley Perrette), conversation with Palmer about the importance of faith, forgiveness and other admirable qualities, he has a change of heart. 

Palmer and Breena then go on the adoption interview, and it goes well. Tony (Michael Weatherly) and McGee (Sean Murray) go out fo ra holiday drink while Bishop (Emily Wickersham) heads to Oklahoma to spend time with her family and husband. 

On the next episode, "Kill Chain," "NCIS partners with the Department of Defense, including Gibbs's former girlfriend Hollis Mann, to track down a stolen drone that was used by a terrorist to kill a sailor. Meanwhile, McGee hesitates to ask for time off to attend a gala honoring his girlfriend."

Showrunner Gary Glasberg told TVLine that McGee's relationship with his girlfriend will progress in the next episode. "Delilah gets really involved in the case, and her and McGee's relationship moves along a bit further," he said. 

When the show returns, a special 250th episode will guest star Robert Wagner as Anthony DiNozzo Sr. Glasberg gave some hints about the episode.

"Tony has had a year of real change and growth so far. Ziva said goodbye. Bishop said hello. Tony's friendship with McGee got stronger and his bond with Gibbs wasn't weakened by change, it was reinforced," Glasberg said. "And now, for a special 250th episode, Anthony DiNozzo Sr. (returning guest star Robert Wagner) will be back. And along with a riveting crime to solve, Tony will discover there's a new mystery woman in his father's life."

"Kill Chain" airs on Tuesday, Jan. 7 at 8 p.m. on CBS.