As the NCIS team investigate an explosion that occurred at an upcoming terrorist summit that was scheduled to take place on a Navy ship and which also resulted in the death of a Navy Petty Officer, Gibbs chooses to sit out the resulting investigation, choosing instead to help grief-stricken FBI agent Tobias Fornell whose life is on the verge of imploding altogether as Fornell still struggles to come to terms with his ex-wife's brutal murder while an old enemy returns for a final showdown with both Gibbs and Fornell.