Later in the series of TMNT, the writers seemed to be trying to phase out Shredder, Krang, Bebop and Rocksteady. This is one of the first episodes not to feature the four regular baddies, and it is probably the best. This episode marks the first appearance of the Rat King, one of several 'guest baddies,' brought in as an alternative menace to Shredder and co. While most of these baddies were never seen again after their debut episode, the Rat King lasted, and was the best of a mostly bad bunch. Most episodes without the regular baddies were often painful to watch, but the Rat King was an excellent baddie, with his bizarre plans of world domination.
Series 3 has some of the best individual Turtles episodes, 'Enter the Rat King' stands out as one of the best. It has the dark setting that is evident in all of the best Turtles episodes, as well as the mood being set perfectly by the music. The dark, rainy traffic jam in the opening scene, Irma searching for April in the sewers alone, Splinter turning against the Turtles under the Rat King's power, and April disappearing in the middle of a live broadcast. An episode with an excellent storyline, a great baddie, and a perfect setting. A fine example of why TMNT was so successful at its peak.