Gibbs is at the murder scene of Navy Captain James Norton only to discover that our victim’s fiancée happens to be his ex mother-in-law Joann, mother of Gibbs' late first wife Shannon.
The last time Gibbs and Joann saw each other was Shannon and his daughter Kelly’s funeral, after Shannon and Kelly were murdered by a Mexican drug carte).
The more Gibbs questions Joann, the more her story doesn’t add up.
Abby and Ducky discover that the bullet trajectory and blood spatter pattern implies that Joann was the shooter, not the random masked gunman she claims she saw.
The team also learns Norton had ties to a drug cartel in Mexico, the same cartel that killed Shannon and Kelly.
Joann hired a PI to help track down anyone related or associated with the Cartel and Norton’s name came up.
Agents track the murder weapon to a Lt. Shankton who coincidentally ran “fishing charters” (transporting drugs) in Mexico around the same time Norton was there.
In the end Joann devised a plan for Norton to get close to her so that she could ultimately hurt him for being connected to Shannon and Kelly’s killers.
In doing so she incriminated Shankton by stealing his weapon to kill Norton, and hiring the P.I. she knew the cartel would sniff out and eliminate themelves.
It was her way of revenge.
Unwilling to take her down, Gibbs secretly enlists M. Allison Hart to represent Joann with the impending murder charges about to be handed to her.
Gibbs gets Joann to confess and arrests her, but Hart says her statement is inadmissible because she wasn’t read her Miranda rights.
We're led to think Gibbs knew what he was doing all along.