Paradigm City is the main setting of the televised anime series The Big O. Located next to a sea and a vast desert wasteland, the city is maintained and controlled by the monopolistic Paradigm Corporation. The city is divided into the higher-income population residing inside geodesic domes, and the remainder living outside the domes. Androids coexist with the human inhabitants of Paradigm City. While their numbers are fairly low and they're something of a rarity, there are enough of them that denizens of the city are not shocked by them and don't treat it as particularly unusual to encounter one.
The Big O takes place forty years after "The Event," an unknown occurrence which left everyone with no memories of their past and apparently decimated the world outside of the city limits. At many times during the series Alex Rosewater, head of the Paradigm Corporation, alludes to the fact that he either does not care about those living outside the domes, or that the city would be better off without them.
The Event makes for the main question of the series, "What happened?" The Event's causes remain unknown, aside from that people's memories and all record of prior events were erased. Memories recovered by Roger Smith and the "tomatoes" (clones of Paradigm City's senators), suggest that an army of Big Megadeī attacked and destroyed the city, before turning on each other.
In reality, Paradigm City is a stage; nothing existed before forty years ago. Enormous machinery underlies the city, and massive stage lamps hang above it. The memories possessed by Roger and the tomatoes are based on a book entitled Metropolis written by the city's founder, Gordon Rosewater, but he says that the story was delivered to him in a dream and that he is not the true author. The final episode reveals that it was written by Angel, who is actually a Rosewater. This alludes to Paradigm City acting as a movie based on this book with the perfect actors, who have no choice but to play the part.
Following the Event, Gordon Rosewater "founded" Paradigm City, and he and the city's senators reconstructed the city and built its domes. It seems that at this point Gordon Rosewater was aware that the city was just a stage, and hired Roger Smith to negotiate with the director, although Roger is unaware of this as he should not even have been born at the time.
Although Paradigm City might be New York City, or an imitation of it, it is much more expansive than the New York City of today. While there are recognizable landmarks from New York City's past, (i.e. the Grand Central Terminal) building apparently expanded in all directions and within the old boundaries itself. Because the city's population only fill about half the city, giant robot battles that cause massive property damage aren't much of a problem, as they either destroy abandoned buildings, or people displaced from destroyed buildings can easily find more that are available.
There is a portion of Paradigm City, that has fisherman's wharf of San Francisco.
Roger travels to various areas of the city to meet with his clients as well as get information from an informant called Big Ear at the "Speakeasy", a bar/pub outside the domes. He also travels to Paradigm Dome for a few jobs, although he despises the company and its headquarters.
The old underground subway and maintenance systems were abandoned after The Event, as everyone was afraid to go below the surface of the city. Many believe, rightly, that hidden secrets from before the Event lurk in these unexplored warrens. Secret laboratories and other installations from before the Event are tucked away in the underground, where none dare look. Roger Smith is unafraid to use the old subway system as a means of quickly transporting Big O around Paradigm City as needed: an immense tank-like vehicle that contains the Big O, the Prairie Dog, uses the subway tracks to maneuver around the city and then raise the Big O on a platform so that it may burst forth from underground. As a result, no one can track Big O's movements to Roger's house. However, even Roger has only mapped out the basic subway grid to use to move Big O around, and even to Roger almost all of the underground is an entirely separate world that he is afraid to go into. In episode 4, Roger ventured further down than he ever had before, and found that the deeper you go the more modern the surroundings become; newer installations were built below the older ones. Thus while the tunnels near the surface are simply old subway lines, the regions very deep down are from right before the Event and house exotic technologies that are unknown on the surface, such as the ruined Archetype megadeus found by Schwarzwald.
Beyond Paradigm City, there is nothing: whatever happened in the Event forty years ago apparently turned the rest of the world into an endless desert, and Paradigm City is believed to be all that remains, to the point that its inhabitants scoff at the idea of surviving "foreigners" as simply myth. Paradigm City itself is essentially a city-state, with fields contained within domes where crops are grown to feed those within, such as the fields of wheat and tomatoes owned by the retired Gordon Rosewater. However, during the course of the series a group called "the Union" surfaces which appears to be made up of foreigners who are infiltrating the city. However, in the surreal final episodes their actual status as foreigners is called into question. Although Union members themselves believe they are really from outside of Paradigm City, it is suggested but not substantiated that they are really clones produced as an experiment within the city, and then cast out, and whoever they think they are working for doesn't exist. Angel learns from her "mother," high-ranking Union member Vera Ronstadt, that they are "tomatoes," Gordon's clones of Paradigm senators who supposedly possess their memories. Gordon later tells Roger and Angel that they are not among his tomatoes, although it seems that his "son," Alex Rosewater, is.
A large portion of the city is also submerged in the waters just off the coast. It appears that during the Event, there was either a massive rise in sea levels, or a large portion of the city sank into the sea, or both, forming a new coastline. Many of the taller buildings can still be seen protruding from the water. The submerged city also acts as a large artificial reef, and attracts a large amount and variety of fish and other oceanlife, which is harvested to help feed the city's population.
In the final episodes of the series, as Roger's megadeus sinks into the sea, he sees below the submerged portion of the city what appears to be a large series of gigantic gears and mechanical equipment below the ruins, lending support to the theory that the entire city itself, even the old submerged ruins, may just be a fabrication. Enormous, megadeus-sized stage lamps are hung from a structure high above the city, reinforcing the suggestion that the city is a stage, although the population is unaware of them as they are concealed by clouds.
Heaven's Day is a secular holiday, which started out as an annual celebration of the founding of the city by giving bread and wine to the poor. As the city grew more prosperous over the course of 40 years, people started giving presents to the ones they loved, and it grew into a commercial shopping extravaganza not unlike modern day Christmas celebrations. The inhabitants of the city still put up generic Christmas decorations like decorated Christmas trees and streamers, but they don't really know the underlying reason behind all of this. However, Alex Rosewater at least knew that the real reason for the celebration was that "it's the day God's son was born". Whether he was referring to a memory of the original holiday, or to himself in comparison to his father is unknown.
The Union's mission is seek out lost memories which they believe may reside in Paradigm Ciy. They also wish revenge on the Paradigm Corporation and the city itself because of their belief that they were responsible for the Event 40 years prior that destroyed the world. They accomplish their goals primarily through subterfuge and sabotage, although they apparently have control of a contingent of Megadeus that they have found in the wastelands, as well as an impressive air force (something that Paradigm severely lacks).
Towards the end of the series, the memories and purpose of the Union are brought into question, and there are indications given that they have been manipulated and that their actions have been lead all along by Paradigm Corporation without their knowledge. There are also indications that their memories of having grown up in foreign lands may be a fabrication, and that their entire lives were just acts on an elaborate stage. If the interpretation is true, it could possibly be that there is indeed, literally, no outside world beyond the wastelands that border Paradigm City.
Angel under the guise of Casey Jenkins, investigator for Paradigm Power Management, re-activates the hydropower plant. Eel emerges and Big O arrives to destroy the giant electric stealing eel. Once destroyed Electric city briefly lights up with electric power discharged from death of giant Eel.
Also, Big O somehow is transported to this remote area. Suggesting that either travelling by underground railroad tracks. Which isn't explained as to the existence of forgotten city in later episodes.