This unnamed treatment plant was an Umbrella facility which was constructed within the Arklay Mountains some eight miles northwest of Raccoon City. It treated the water released by Umbrella's executive training school, the laboratory of which was closed in 1988. The treatment plant and an adjacent factory were left to deteriorate for the next decade, though were still operational until July 1998. The facility was destroyed on 24 July to contain a biohazard.
The treatment plant was constructed by Umbrella to process wasted generated by the company's isolated collage, run by Dr. James Marcus, taking advantage of underground water channels. Starting from 1978 or later, Dr. Marcus began dumping the corpses of failed human test subjects of his t-Virus experiments in the channels, and some of which washed onto the lower levels after flooding. Following his assassination in 1988, Dr. Marcus' body was dumped in the same fashion. Over the next ten years, the Queen Leech test subject dumped with him consumed his body and gained his memories, ultimately leaving the plant to seek revenge for its creator's death, believing he and it to be one and the same.
Despite the shutdown of the training school and its lab, the treatment plant continued to process waste, perhaps from other sources. Starting from 10 May 1998, the facility began processing an unexpected increase in industrial waste, which the facility was not built to process. On the early hours of 24 July, the facility was ordered to evacuate after Drs. Albert Wesker and William Birkin discovered the Queen Leech was responsible for a series of biohazards in the region. The staff were attacked during the evacuation, however and mutated into Zombies. Later that morning, Rebecca Chambers and Billy Coen found themselves in the facility, and were forced to fight the Queen Leech in order to escape from the explosives activated to contain the outbreak.
The treatment plant was built within a series of large, naturally-occurring limestone caverns in the Arklay Mountains, and processed waste water dumped into underground river channels by Umbrella facilities. The treatment plant was directly connected to an abandoned factory, which was of a similar design to the Underground Laboratory in Raccoon City's industrial zone. A cargo elevator led up to a large warehouse built near the executive training school.
The complex itself was largely made up of sewage pools and water treatment tanks, but also contained staff dormitories as well as a bar and recreation lounge; however, by the time Rebecca and Billy entered the plant the dank and gloomy facility was mostly home to zombies, lurkers and other by-products of Marcus' experiments.
The Treatment Plant's layout as depicted in The Umbrella Chronicles'Train Derailment 3 is somewhat abridged; the elevator Billy and Rebecca use after their encounter with the T-001 takes them directly to the generator room located next to the alley leading to the incinerator chamber, whilst in Resident Evil 0 the elevator opens out onto a small walkway leading to the main power supply control room. In Beginnings 1 the layout is different again - when Wesker uses the elevator it connects directly to the power supply control room and leaves out the walkway. These changes were presumably made in order to cut down the length of the level for pacing reasons.
Despite containing the largest number of Leeches in any area (in the incinerator room) in the game, only one Mimicry Marcus is located in the entire Plant. However, it is the fastest one in the game.
Due to the geographical issues of the so-called "magic elevator", which allowed players in the factory to return to the training school to collect items, it is uncertain exactly how close the factory and treatment plant are to the school, though the game's ending certainly puts its warehouse near the Spencer Mansion.