On Saturday April 27 at 10 am ET, MeTV will air Season 4 Episode 12 "The Night of Miguelito's Revenge". Original air date December 13, 1968.
The above recap is from the IMDb page for the episode.
Ross Martin does not appear in this episode as he was recovering from his heart attack. The script has Artie on assignment in DC.
This is the 10th and last appearance of Dr. Loveless. At the end of the episode, Loveless vows to return again. Alas, it was not to be.