Mounted Police Sergeant May Dawson is out on patrol in Rock Creek Park one night when she comes across the dead body of Marine Gunnery Sergeant Remi Tanner.
After NCIS is called in, she also discovers some bloody burlap, which reveals a collection of animal skulls. Curiously, they belong to a species of monkey found in Liberia, where Tanner had been training wildlife wardens in anti-poaching operations.
Sgt. Dawson joins forces with the NCIS team when another mounted officer is fatally wounded, almost certainly by the same person who killed Tanner.
Meanwhile, the Liberian ambassador shows a suspiciously keen interest in the case and appears to know more than what he's saying.
The team's investigation leads them to a cabin in the woods, where they find a stash of what is called "bushmeat," illegally hunted meat that has been smuggled from Liberia.
The ambassador confesses that he suspected such things were afoot, and offers any assistance he can to bring the killers and smugglers to justice.
He takes Reeves to an open-air market to search for possible vendors of the illegal bushmeat, but they aren't able to find anything new immediately.
The situation becomes more serious when it is announced that there is an Ebola outbreak in the region of Liberia where the bushmeat is being harvested -- any new shipments will likely be contaminated.
The team finally discovers that Tanner had come across the smugglers by complete accident and had recognized one from a photograph. He had followed them and taken photos, but they caught and killed him.
After the arrest, Gibbs takes out the fallen mounted officer's horse Jody for a ride with Sergeant Dawson.