Secrets arise when Gibbs former mother-in-law surfaces as a witness in a murder investigation.

After a mysterious illness strikes children from military families, Gibbs and the team investigate the cause. Meanwhile, Abby and Jimmy partner with the Naval Medical Research Center to determine the strain of illness, all in hopes of delivering a cure before the holidays. Also, Director Vance struggles with forgiveness as his estranged father-in-law makes a surprise visit to his home.

Gibbs and the team investigate when a Navy lieutenant who is set to be the first openly gay servicemember to receive the Medal of Honor is murdered. Also, as his wife's due date approaches, Palmer prepares for fatherhood with an NCIS baby shower. Rocky Carroll makes his directorial debut.

While investigating the death of an intruder at Quantico, Jimmy and Kasie are exposed to a deadly biotoxin; the NCIS team calls on Dr. Carol Wilson for help as they race to find the antidote; Torres tries to distract Jimmy's daughter.