NCIS opened with a Captain Mason Green leaving the performance of The Nutcracker with his daughter and son-in-law. When he gets into the car he scheduled to pick him up, the driver offers him a bottle of water and watches as the captain begins coughing. He drives to an abandoned area and takes the deceased captain out of the car before driving off.
At Gibbs’ house, Emily wakes her father, who is still living on his couch. Fornell tries to make excuses for why he is not exercising when he picks up on the fact she has a boyfriend. Gibbs gets out of the conversation when he gets the call about their case.
Alex and Tim investigate the crime scene and discuss a witness statement about him getting into a rideshare before his death. As Ducky and Palmer arrive, Alex hands Tim a piece of paper with an address on it. It turns out to be Ducky’s old home.
The victim’s cell phone is missing but Abby is working on trying to figure out what rideshare service he used. Tim and Alex tell Gibbs that Green was under investigation of selling classified information. When Alex thinks it is a coincidence that Green had Ducky’s address, they end up discussing the rules before Gibbs leaves. They are going to see if Green was actually looking for the current owners and not Ducky.
In autopsy, Ducky tells Nick he does not recognize Green before Gibbs finds out from Jimmy there was a poison in Green’s system. Ducky does not have any files on Green and he is asked if he met Green during his travels. When he finds out the captain had been in Majorca, Ducky recalls a story about the place.
Ducky has a memory of walking in on his mother and a male friend of hers, Balthazar Kilmeany. He had thought she was in Majorca and had brought a female friend over. His mother had told him they were in love. He loses himself in the memory and had not told the others about it.
Gibbs and Ducky talk to Green’s daughter and son-in-law. Neither recognize Ducky or the address. The man tells them Green had been asking about if he had any connections overseas but she is adamant her father did nothing wrong. Ellie and Tim come to let Gibbs know that Green was actually looking for Victoria Mallard, Ducky’s mother.
Vance and Gibbs interrupt Nick and Clayton arm wrestling to discuss his conversation with the Secretary of Defense. They are hoping to find out who is involved with the possible stolen intel, especially after finding out Green had a bank account in the Crimea.
Tim and Alex talk to Ducky’s former gardener, who was asked to stay after Ducky sold the house. He tells them Green had come with a friend of his but he could not see who he was clearly. When Tim spots a door camera, they get footage to see the car had no plates. Then Abby comes to tell them that after Tim and Alex left, the gardener was quick to get away from the house. Abby asks Ducky for tissue samples before heading to the lab.
Emily interrupts Gibbs to ask if she can do whatever she needs to do to get her father home. He agrees as long as she does not burn down his house. Ducky and Emily reminisce about their mothers.
Ducky recalls in his younger days, he had asked his friend Angus to look into Balthazar since his mother was now engaged to the man. In the present, Abby tells Gibbs that Green’s rideshare account was infected with a worm to send his ride request to a third party. As Ducky comes in, Abby tells Gibbs that Green made a phone call to Balthazar Kilmeany, leaving Ducky stunned.
In the interview room, Ducky tells Vance and Gibbs he would be surprised of Balthazar Kilmeany was still alive. In addition, he informs them the man was actually a con and he had ordered him to stay away from his mother or he’d go to Scotland Yard.
Back in Gibbs’ basement, Fornell is trying to get Gibbs to run a background check on Emily’s entomologist boyfriend. Gibbs is distracted though when he sees a termite on his boat. He gets a phone call from Tim about the case as Fornell calls an exterminator.
The BOLO came back on the driver who took Green to Ducky’s old house. While his prints are still being run, Emily and Alex interrogate him as Gibbs watches with Tim. The man is actually a private investigator and Green had been looking for Balthazar Kilmeany. While he did not have anything to do with the murder, he does have a hard drive Green had given him for safe keeping.
Ducky is surprised his mother had taken a personal ad out looking for Balthazar but surmised she was trying to recapture happier memories. They also discover Balthazar Kilmeany was a name used by multiple people as an alias. While they are still trying to figure out the case, Abby tells them that Green was innocent and never sold classified data.
Tim and Ellie tell Gibbs that Green had proof he had been set up for the stolen intel and someone opened the account under Balthazar’s name. As they work on the BOLO for the gardener, Gibbs discovers his house has to be tented for termites. At first, he is angry but then realizes this means Fornell has to leave.
Alex tells Ducky that they picked up the gardener but he is not in the mood to discuss anything. After she leaves, he has a discussion with his younger self over whether they made the right choice in sending Balthazar away. He remembers how distraught she was to lose him and that he had asked her to come to America once Ducky was finished working in Pakistan.
The gardener, it turns out, had a romantic relationship with Victoria and at her last Christmas before she died, she thought she saw Balthazar at the theater. They discover Green’s son-in-law, Logan, was actually the nephew of the Balthazar that Victoria knew. The name was used in the family as an alias. He had sold the data to Anton Vlastefin, who was the killer. Logan claimed he never realized Green was in danger of being killed.
Ducky then tells Gibbs and Ellie he read through his mother’s diary and he was relieved his mother had spent the last years of her life in love. Along with the team and Mr. Rin, Ducky enjoys Christmas with everyone.
At their home, Fornell and Emily talk things out. After claiming he was tired of living with Gibbs, Fornell was stunned to find his friend was now sleeping over with them on account of termites.