As Washington D.C. gets ready for Christmas, the NCIS team find their holiday plans being put on hold as they investigate when a mysterious illness strikes young children from military families with the disease quickly spreading. As the team race to stop a devastating outbreak, Abby and Jimmy work with the Naval Medical Research Center to determine the strain of illness, all in hopes of delivering a cure before the holidays begin while Vance struggles with forgiveness as his estranged father-in-law makes a surprise visit to his home.