McGee used a manual typewriter to write a letter to his just-deceased father as a means of reflecting back on his life, Gibbs' rules, and the NCIS team's latest case involving a cyber-shutdown of Washington.
Gibbs learns that Washington's internet has been shut down by a cyber-terrorist group lead by a man who calls himself Krampus. Just a few hours before the team was scheduled to go on holiday leave, he calls them back to the office to investigate.
It turns out the internet shutdown was done in order to distract from the real goal: an attack on a government server in order to extract vital security intel.
They discover the body of one of the hackers - a man who had a heart attack, as a complication of a condition he had, called "fatal familial insomnia".
The team brings in a group of three convicted cyber hackers from prison to assist in locating the leader of the group responsible for the Washington cyber attack. One of them is former NCIS Systems Administrator Kevin Hussein.
Bishop discovers that one of the hackers - Heidi Partridge - has been in contact with Krampus.
Gibbs and McGee come up with a plan to gain information on Krampus' whereabouts: they offer a deal to one of the convicts, Ajay Khan: pretend to have stolen Bishop's cellphone so that he could give it to Heidi. The NCIS team would then monitor any phone calls with the intent of identifying Krampus' location.
it didn't work: Khan instead told Heidi all about the plan.
Bishop finds a picture of Krampus on a social network site and comes up with a possible location for him. McGee distrust the intel - he thinks the information came to them too easily. Gibbs agrees and tells McGee to appeal to Heidi.
McGee is fed up with all of the convicts and doesn't trust them. He doesn't think it could work. Gibbs reminds him of Rule 51: sometimes you're wrong.
McGee confronts Heidi and appeals to her as the teacher she used to be. She gives up Krampus' location and federal agents storm the Jacksonville Retirement Community where he is staying with his parents. They arrest him.
In the final scene, McGee is reading his typewritten father to his deceased father. He places the letter into the coffin.
The end credits display this message: Happy Holidays to the men and women who protect and service our country at home and abroad.