Ducky gets an urgent call from a guy claiming to be a lawyer, saying that he had information about his half-brother Nicholas Mallard, and asking to meet. He agrees and rushes out the door.
The team is called to investigate the stabbing death of Marine Private John Angel.
Gibbs asks Palmer where Ducky is. Palmer tells him he has no idea and has been trying to call him all morning. Gibbs tells him to try again.
They hear the sound of Ducky's ringtone coming from a nearby van. Upon opening the door they see Ducky bound and gagged on the floor of the van.
Ducky tells Gibbs that the "lawyer" he met had no information but was instead fishing for information from him about his brother. When he realized what was going on, Ducky left the meeting.
Ducky says the man followed him and tried to push him into the van in which he was found. He struggled but was knocked out.
Ducky says the dead Marine had been in the coffee shop, but was not the "lawyer" he met.
Flashback: Nicholas was 20 years younger than Ducky.
Ducky and Nicholas' step mother Lorraine decides to go out of the house for a while, despite Ducky's protest that it's Christmas Eve and that she should stay and be with Nicholas.
Young Ducky gives Nicholas a train set for Christmas.
Present: Ducky tells Gibbs that Nicholas passed away years ago, before he and Gibbs even met.
Tony and McGee tell Gibbs that the van in which Ducky was found was rented out via a smartphone app called "Pimp Your Wheels." This is a service that allow private car owners to rent their vehicles out to strangers. The owner of the rented van is Sam Butler.
Gibbs tells Bishop that Jay called, and that he planned to come over and talk with her.
Flashback: Young Ducky's at the military hospital, looking for Field Marshall Rand.
The nurse tells him Rand specifically asked for Ducky and wouldn't let anyone else treat him. He kept shouting for Ducky until he passed out.
Young Ducky's father shows up in the ER. He tells Ducky that Lorraine came by his work and served him divorce papers, and is leaving the country. She told him she's taking Nicholas with her.
Present: McGee and Tony interview Sam Butler, the owner of the van.
Butler tells them he rented the van out to Rufus Simms. McGee shows him a composite picture of the man who attacked Ducky. He says it isn't Rufus, but it looks like a guy who was with Rufus last night. He tried to rent him his bicycle.
He tells them Rufus rented his house as well.
A girl screams. Tony, McGee and Butler rush to a window where they see Rufus' dead body hanging in the living room.
Ducky tells Gibbs that Rufus drowned 12 hours ago. His body was later staged to look like a suicide.
Bishop tells Ducky that an old letter to Ducky from Nicholas was found in the lining of Rufus' suitcase. Ducky never saw it before. The letter was addressed to Ducky's old flat in London. He hasn't been there in forty years.
Flashback: Young Ducky meets young Angus Clarke in a pub. He tells Angus he needs his help. He wants Angus to help fight for custody of Nicholas.
Angus tells him Ducky's father won't be able to look after him. Ducky tells him he's resigning his commission in order to take care of his brother.
Present: Jake shows up. He asked to speak with Bishop.
Bishop and Jake go to the garage to talk, not realizing that McGee is there too, inside the van dusting for fingerprints.
Bishop and Jake discuss their marriage. Bishop tells him it's over and has been for quite a while.
Abby tells Gibbs she can track Rufus' movements prior to his death because of data found on his smart watch. She tells him she can ascertain what he was typing on a computer.
She tells him he was doing a Google search for a pawn shop, specifically a place called "Parker's Pawns."
Abby tells Gibbs that the letter Nicholas wrote was done three weeks ago, which means Nicholas is still alive.
McGee tells Ducky that DNA pulled from the letter has a familial connection to Ducky.
Tony and McGee visit Dwayne Parker at his pawn shop. Parker tells them that Rufus pawned a stamp for $2000. Unfortunately the stamp was damaged because someone tried to mail it, otherwise the stamp would have been worth more than $500,000.
Parker tells them that the guy Rufus was with got into a fight with him over the value of the stamp.
Parker provides them with a picture of the guy they're looking for, the man who was with Rufus in the pawn shop.
They identify the man as Viggo Trellis a Philadelphian private investigator. He had recently been hired by Rufus Simms.
Bishop shares their theory with Gibbs: Nicholas used an obscure collector's stamp to mail his letter to Ducky. The stamp wasn't valid, so the letter ended up in the dead letter office where Rufus was working as a manager.
Rufus realized Nicholas didn't know the value of the stamp, so he hired Trellis to hunt for Nicholas. When Trellis found out the value of the stamp, he killed Rufus and went looking for Nicholas himself. That's when he contacted Ducky.
Flashback: Ducky goes in for a final medical consult with his patient Field Marshall Rand. Rand tells him that Nicholas dropped his stamp book on the night in which he was admitted.
Out of admiration for Nicholas, Rand filled the stamp collection book with some sample stamps from his own collection.
Young Angus tells young Ducky that he and his father can't win the custody battle, which is why he's giving them the 10,000 pounds to buy off Nicolas' mother.
Young Ducky receives a call at the hospital. A neighbor has spotted movers at the house. Ducky is worried that Lorraine is trying to sneak away with Nicholas. He rushes out of the hospital.
Present: Bishop is able to zero in on Trellis' phone. She turns on the microphone so that everyone in the bullpen can hear what's going on.
Trellis has a captive driving, directing him to a location. Trellis refers to the driver as Nicholas.
NCIS puts out a BOLO for Trellis.
They hear noises that indicate Trellis has gone back to Butler's rental house. Gibbs, McGee, Tony and Ducky rush to the rental house.
Flashback: Young Ducky arrives back at the house. Lorraine tells Ducky that she no longer wants the money. She's taking Nicholas with her. She's arranged for police to guard her as she takes him.
Young Ducky tries to intervene, but the police hold him back.
Present: Ducky hears a series of gun shots in the house. He rushes in to see Trellis dead on the floor and a man bound to a chair with his mouth taped shut. Tony removes the tape from his mouth.
Flashback: The police are still holding Young Ducky as he struggles to get Nicholas back. He punches the bobbies, but the car drives away. The bobbies arrest him as he shouts in horror at the fleeing vehicle.
Present: Ducky walks up the man identified as Nicholas. Ducky says he's the wrong Nicholas. Tony takes the man away.
Flashback: Young Ducky's in the living room with his father. Young Angus walks in.
Angus tells Ducky that he believes Lorraine took Nicholas to Albania. Ducky vows to go find him.
Present: Ducky tells Palmer that one of Lorraine's relatives told him that Lorraine and Nicholas were killed in a car crash in Prague.
The rest of the NCIS team enters the room with papers showing that Nicholas not only is still alive, but that his mother arranged for a name change. They tell him they found him, living with early onset Alzheimer's and living in a nursing home.
Ducky and Bishop arrive at the nursing home. Ducky walks up to Nicholas and scratches his head as he used to do when he was a kit. Nicholas beeps Ducky's nose and smiles.
Nicholas recognizes his brother. After all of those years apart, the brothers finally find each other and embrace.