Ducky gets an upsetting call, and agrees to meet with the caller at a coffee shop without telling anyone where he is going or why.
The NCIS team is alerted to a dead Marine in an alley. When Jimmy Palmer shows up alone at the crime scene, Gibbs wants to know where Ducky is. Palmer says he tried calling Ducky's cell 11 times but he never picked up, so he came without him.
Palmer tries Ducky's cell one more time, and Gibbs hears it ring. Ducky is tied up with duct tape in the back of a blue van at the crime scene.
Ducky explains that he got a call from a man claiming to be a lawyer who said he had information about a member of his family. When they met at the coffee shop, it turned out the man had no information and was fishing to get some from Ducky, so he left. The man forced Ducky into the van and knocked him out. The Marine was also in the coffee shop, and probably saved Ducky's life at the cost of his own.
Gibbs wants to know who the "lawyer" was asking about. Reluctantly, Ducky tells Gibbs about his much younger half-brother, Nicholas Mallard. He explains that his stepmother wasn't exactly Mother of the Year and that Nicholas passed away years before Ducky met Gibbs, which is why he never brought his relative up.
Ducky recalls the final, emotional days he had with Nicholas as a young man home from the Royal Army Corps. His step-brother was 8. Nicholas' mother, Ducky's stepmother, told him he could watch Nicholas while she went out. Ducky gave Nicholas a Mallard train when he got a call to report to the Military Hospital. He took his brother with him. Later, Ducky's dad came by the hospital and told him that Lorraine filed for divorce and was taking Nicholas with her.
DiNozzo and McGee track down the blue van. It was registered with a company called Pimp Your Wheels, which allows everyday people to rent out their cars to strangers. Bishop gives them a sketch of the "lawyer" and they head out.
Gibbs warns Bishop that Jake is on his way over. Jake tells Bishop that he is sorry. She says that doesn't help. Jake says his affair was a symptom of something deeper. When they worked together at the NSA, they were happy in their bubble, but when she left, there was a wall and they couldn't talk without violating national security and that was the problem. He promises to do whatever it takes to repair their marriage. Bishop says they weren't working and the marriage is over.
McGee and DiNozzo track down the owner of the van and inform him it was used in a crime. He tells them the van was rented to a man named Rufus Simms. He's not the guy in the sketch, but he did see that guy with Simms. He also rented out his house to Simms, who is found hanging from a rope.
Simms' death turns out to be murder staged to look like suicide. Ducky doesn't recognize Simms, who was a department manager at a post office in Philadelphia.
Bishop tells them a search of Simms' suitcase turned up a letter hidden in the lining. It's from Nicholas. Ducky says he never saw the letter, and he notes that it has to be more than 40 years old.
We return to the flashback, where Nicholas is worried about his mother taking him away and Ducky not being able to find him. Ducky tries to get legal advice, because Lorraine says she will give him custody of Nicholas for ten thousand pounds. Ducky tells his lawyer that he plans to resign his commission in the army and raise Nicholas.
Abby is able to reconstruct a 3D map of Simms' movements from a gadget he had on his wrist. She was able to figure out that he Googled a place called Parker's Pawns before he died. She also discovered that the letter was written three weeks earlier, which means that Ducky's brother is still alive. Ducky initially doesn't believe it, but Abby was able to retrieve touch DNA that is a familial match.
DiNozzo and McGee go to the pawn shop and learn that Simms was there to pawn a rare stamp, one that came from a collection Ducky had given Nicholas. While there, they get a video of the "lawyer." He turns out to be one Viggo Trellis, a private investigator from Philadelphia.
It turns out that Simms hired Trellis to find the person who mailed the letter with the valuable stamp in the hope that he would have more. When Trellis found out how much the stamp was worth, he killed Simms and searched for Nicholas on his own.
McGee finds Trellis' cell phone, but it doesn't have a GPS. Ducky tells them to turn it on, and they hear Trellis in the car with Nicholas. They determine that he is back at the rental house and head there. Trellis is killed, but the Nicholas Mallard he found turned out to be the wrong guy.
In the flashback, Ducky vows to scour the planet for his brother.
Ducky tells Palmer that a relative told him Nicholas and Lorraine were killed in a car crash in Prague. He says the news almost killed him and he doesn't know if he has the strength to start that journey again.
The NCIS team brings Ducky Christmas presents that lead to Nicholas' location, a nursing home just outside Philadelphia. Nicholas has early-onset Alzheimer's, but is otherwise healthy. Ducky is reunited with his half-brother, who remembers him when Ducky gives him the Mallard train.