After spotting the brother of an NCIS Most Wanted suspect, Gibbs immediately assumes a former alias and goes undercover with their infamous anti-government militia group while Bishop and Reeves work together on an off-the-books investigation.
This is the 319th episode overall, airing before the 316th.
Gibbs' alias, Leland Robert Spears, is the same alias that was stolen in the Season Twelve episode, Blast from the Past (episode). This was also given a reference in the beginning when the terrorist mentioned that he thought Spears was dead.
It is unclear whether the undercover case Gibbs was working on to catch the Whitmans' in 2001 is before or after the undercover case he was on in Baltimore in 2001, that involved him meeting DiNozzo.