The team is now led by Di Nozzo for a short time until Gibbs' eventual return.
New characters introduced in this season are Michelle Lee, who was briefly on Di Nozzo's team and was transferred to the legal department upon Gibbs' return, and (already in the final episodes of season 3) Gibbs' former boss and mentor Mike Franks, both as recurring characters.
The investigation leads them to a fact that the couple has been separated.
They decide to focus on the wife's ex-boyfriend, after learning that the woman is the one who destroyed their marriage.
The team soon finds discrepancies in the petty officer's case and that he may have been framed.
Abby runs the DNA samples from the two women to find out if the DNA is a match to the soil found on the dead man's body and later finds out that the team are looking for a third woman who might have been sleeping with the victim.
It is Halloween, and the NCIS team is busy investigating a ransom case in which a Marine's daughter has been kidnapped after the kidnapper attacked the Marine in his home.
A decorated Marine veteran falls from a floor of a hotel during a conference of agency directors (that included Jenny Shepard) while Tony and Ziva were on security detail and the NCIS team must find out what happened to him.
Soon they realize that the Marine did not commit suicide and that he was homeless, having lost everything after being injured while serving in Iraq.