While Ziva, undercover, goes on a date with a suspected serial killer (an enlisted Marine), Tony enters the suspect's home and finds the body of the fifth victim; Gibbs and McGee pursue. Meanwhile the suspect threatens Ziva, who resists him and shoots him to death with his own piece; she experiences a strong emotional recoil. The team continues to investigate; Tony and Ziva go to the bar where she had met the guy, and she meets another man. Abby questions whether the dead guy acted alone, and she thinks there may be a sixth victim. Tony and Abby find some new evidence; the gang resolve the questions and apprehend the second killer. Finally, Ziva gives a favor to a friend.