Moonside is an area in EarthBound. It is only accessible through the rear wall of Jackie's Café in Fourside during a certain point in the game's story. After Paula has been abducted from the Fourside Department Store by a minion of corrupt Fourside mayor Geldegarde Monotoli, Ness and Jeff stumble upon Mr. Everdred lying injured in an alleyway next to the Café. Everdred explains how Monotoli betrayed him to acquire the Mani Mani Statue, which Everdred had previously stolen from Carpainter in Happy Happy Village after the cult leader's defeat by Ness. Everdred also reveals that Monotoli receives "evil power" from the golden statue, and that the mayor keeps the Mani Mani behind the counter inside Jackie's Café.
By checking the wall behind the counter, Ness and Jeff are suddenly transported to the world of Moonside, a dark reflection of Fourside where the inhabitants speak gibberish and surreal enemies roam the streets. Positive and negative dialogue choices are reversed in Moonside; yes means no and vice-versa. Navigating this area can prove difficult due to numerous invisible barriers which cordon-off certain sections of town. However, the party can speak to a number of "Warp Men" positioned around Moonside to be teleported to a different area. These warps eventually lead Ness and Jeff to Monotoli himself as he cowers before the Mani Mani and suddenly disappears. Checking the golden statue begins a boss battle against the Mani Mani, which assaults the party with powerful PSI attacks until it is defeated.
Once the evil idol is destroyed, the illusory world of Moonside also disappears. A nearby mouse explains that Ness and Jeff had simply been wandering around inside a large warehouse behind Jackie's Café during their "trip" to Moonside.