A grisly discovery is made in a dumpster in small West Virginia town: a severed leg with a sock pulled over the foot. And since the limb bears a Semper Fi tattoo, Gibbs (MARK HARMON), Tony (MICHAEL WEATHERLY) and Kate (SASHA ALEXANDER) are called to the scene to investigate, with Ducky (DAVID McCALLUM) examining the leg for medical clues. At first glance, all Ducky can ascertain is that the leg would have been severed from a live body less than 24 hours ago. But with an X-ray, he can tell that the heel contains a titanium ankle which – containing a serial number – can be traced to the doctor who implanted it. Meanwhile, Abby (PAULEY PERRETTE) tries lifting fingerprints from the limb and finds the seed of a sycamore tree in the victim’s sock.
When Gibbs tracks down the doctor who performed the surgery, he identifies his ankle patient as PFC Thomas Dorn... who, according to military records, supposedly died two years ago from a heart attack. The NCIS team learns that a doctor in the small West Virginia town of Harmony signed the death certificate, but that no autopsy was performed despite Dorn’s young age, so Gibbs sends Tony and Kate to interview her. Upon meeting Dr. Sylvia Chalmers, they find a kindly, about-to-retire medic who says that Dorn came into her office complaining of chest pains. Soon afterward, he keeled over. While she had planned to perform an autopsy, his only living survivor – a half- sister – begged her not to and, resistant to make her loss worse, Dr. Chalmers decided to forego one. The body can’t be exhumed now, because the sister wanted him cremated. And, when the agents show the doctor a picture of Dorn, she says that’s not the man who came in complaining of chest pains. Now suspicious of the half-sister, Melissa Dorn, and uncertain who suffered a heart attack posing as Dorn, Gibbs and Kate pay her a visit at her home in Maryland. But their visit to the sultry woman yields little clues... until Gibbs spots a sycamore tree on her lawn and thinks she must be connected to the deaths somehow.
Next, the agents research a policy that Dorn took out before he died. The beneficiary – Melissa – received $750,000, despite the insurance company’s misgivings about the untimeliness of Dorn’s death. Now that the team has a motive, Gibbs visits Dorn’s Marine gunnery sergeant to determine more about his past, and learns that he and his best friend, Corporal Morgan, were both hanging with Melissa, who didn’t seem like a half-sister to either of them.
Armed with this information – and the fact that the sycamore tree from Dorn’s sock genetically matches the tree at Melissa’s house – the agents return to her home with a search warrant. Just as Tony finds gruesome etchings on a barn floor that correlate with a power saw cutting up a body, Gibbs and Kate realize that someone else is in the house. Soon after, they discover an unlikely accomplice for Melissa: Dr. Chalmers... who, unexpectedly, also happens to be Melissa’s mother. Despite Sylvia’s assertions that nothing more than insurance fraud has been committed, Melissa tearfully admits to the scheme that Sylvia has masterminded. First, the doctor caused Morgan to have a heart attack with a lethal dose of digitalis, claiming he was Dorn in a move that gave them an insurance windfall. Then, to keep the ruse going, she later killed and cut up Dorn.