We are exposed to a new series of strong events in the Fourth season. Mordechai and Rigby must all return to their past so that they can prevent the construction of a highway over the park. Also, Mordecai and Rigby are trying to get Muscleman to stop joking with Thomas because his dumps have become so ridiculous.
In its way, "Exit 9B" is a celebratory episode, a reward for longtime fans and an affirmation that the show's characters have been worth getting to know.
