Ghoullem and Zugg, the ghost gangsters from "Scareface", return from the South Pole to get revenge on Slimer. They put the Ghostbusters to sleep with a sleep-walking potion, and Slimer is the only one who can rescue them (which is just what Ghoullem and Zugg want). However, Slimer doesn't go unprepared, as he gets all his friends to help him.
Visual Commentary for episodes S01E20, S01E17 and S01E23.
Visual Commentary for episodes S01E21, S01E25 and S01E13.