The team investigates when a Navy master diver is killed in the ocean. They learn that he was run over by a small submarine, the type used by drug smugglers. Homeland Security Agent Russo, in New Orleans to set up a DHS office, offers his assistance, having located the submarine.
DHS suspects that foreign assets may be planning an attack in the country, but they have no idea what the target is. The NCIS team locates the two operatives, who are playing husband and wife. The wife, who was injured on the diver's boat by his Marine friend, is captured, but her husband escapes.
From all indications, the operatives really were in love, and even trying to have children. The husband attacks the NCIS office in an attempt to rescue his wife, but is fatally wounded. The team discovers that he was handling explosives, and a lot of them. They were smuggling them into the country for the impending attack.
The team turns over the wife to Russo and Homeland Security for further interrogation. She tells Russo that she's not going to talk. After turning off the video camera, Agent Russo reveals that he's her contact, and gives her a pill.