MI-6 Officer Clayton Reeves asks for the assistance of the NCIS team in locating a missing MI-6 agent, Mason Finley, when Reeves discovers the body of a murdered petty officer at Finley's last known location in Philadelphia.
Finley was looking into smuggled Russian rocket launchers when he went missing. Gibbs dispatches Bishop and a less-than-enthusiastic Quinn to Philadelphia to assist Reeves in the investigation.
The team discovers that the unfortunate petty officer was in the wrong place at the wrong time, a suspicion that is confirmed when they locate Finley, beaten and bloody, after he sent a text message to Reeves.
Investigating a possible lead, the team is shocked when Finley actually shoots the suspect. Furious, Reeves relieves Finley and sends him back to his hotel room.
Back at NCIS headquarters, a stir-crazy Torres finally figures out a hole in Finley's story about being held captive for days, and they all realize that Finley is dirty and in on the smuggling.
They arrest Finley and the smugglers and recover the rocket launchers.
Meanwhile, Gibbs forces Quinn to confront her guilt over a past incident in Philadelphia which resulted in the death of a friend and Quinn's retreat from the field into teaching. Though resistant at first, Quinn eventually gives in and explains everything, and comes out better for it.