This treatment plant was built some eight miles outside of Raccoon City, located adjacent to the abandoned factory and underneath the Umbrella Executive Training Center. In use since at least 1978, it was used by Dr. James Marcus as a dumping ground for at least a dozen failed human experiments - early trials for the t-Virus.
The treatment plant was tasked with the treatment and disposing of the waste generated by the experiments conducted in the Umbrella Executive Training Center above. The plant was frequently used by James Marcus to dispose of his human test subjects; perhaps as 'poetic justice', his corpse would later be dumped in exactly the same manner following his assassination in 1988 by William Birkin and Albert Wesker.
In May 1998, Marcus' final experiment, the Queen Leech, succeeded in 'reviving' the doctor by merging its genome into his own. The treatment plant suddenly became a site of intense waste dumping, with regular and 'contaminated' (implied to be virally-contaminated) waste making its way into the facility in far greater quantities than it could handle. On the early hours of July 24, Umbrella finally gave-up and closed the plant, much to the relief of the staff. Unfortunately for them, they were killed on infected along the way when the plant and parts of the connecting factory were infested.
The leeches, which had continued to breed and multiply during the intervening years, infested the plant's central incinerator chamber and surrounding areas; it was here that Rebecca Chambers and Billy Coen confronted the Queen Leech before managing to destroy him once and for all. The plant was finally destroyed for good along with the rest of the facility when Birkin activated the complex's self-destruct device in an attempt to deal with the Queen Leech (who had been using the identity of Dr. James Marcus).
The treatment plant was built within a series of large, naturally-occurring limestone caverns situated beneath the Umbrella Executive Training Center. The two, along with the Raccoon City Underground Laboratory, were connected via an elevator which could also access the Research Center above. A loading elevator was also used in the Treatment Plant, leading up to a warehouse independent of the Research Center. Following the destruction of the plant the area leading to the elevator was sealed off, and by the time the Raccoon City Destruction Incident occurred the area was completely inaccessible. The plant was also connected to an abandoned helipad which was located somewhere near the Spencer Mansion.
The complex 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 Zero 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.