Resident Evil Zero is a survival horror video game developed and published by Capcom and originally released for the GameCube in 2002. It is a prequel to Resident Evil (1996) and its remake (2002), covering the ordeals experienced in the Arklay Mountains by special police force unit, the S.T.A.R.S. Bravo Team. The story follows officer Rebecca Chambers and convicted criminal Billy Coen as they explore an abandoned training facility for employees of the pharmaceutical company Umbrella. The gameplay is similar to other Resident Evil games, but includes a unique "partner zapping" system. The player controls both Rebecca and Billy, switching control between them to solve puzzles and use their unique abilities.
Development for Resident Evil Zero began for the Nintendo 64 in 1998. The partner system was created to take advantage of the short load times possible with the capabilities of the Nintendo 64 Game Pak. The cartridge format also provided limitations, as the storage capacity was significantly less than that of a CD-ROM. The team had to approach the design differently from previous series entries to conserve storage space. Resident Evil Zero was designed to be more difficult than previous Resident Evil games. Inspired by Sweet Home (1989), the team removed the item storage boxes present in earlier games and introduced a new item-dropping feature. Development slowed when the team encountered memory storage problems, and so production was moved to the newly announced GameCube. The game had to be rebuilt, with only the concept and story carried over.
Resident Evil Zero received generally positive reviews. Critics praised the graphics and audio for building a haunting atmosphere. Opinions on the new partner and item systems were mixed. Some found the changes were an improvement and added new layers of strategy; others believed the changes were cumbersome or non-innovative. The controls were panned as outdated and Capcom was criticized for not evolving the series' tank controls. The game was ported to the Wii in 2008, and a high-definition remaster was released in January 2016. These re-releases received mixed reviews due to lack of improvements made to the original. Resident Evil Zero was commercially successful, having sold over 4 million copies across all platforms.
On July 23, 1998, a train owned by the pharmaceutical company Umbrella, the Ecliptic Express, comes under attack from a swarm of leeches. As the passengers and crew are attacked, a mysterious young man watches over the resulting chaos from a hillside. Two hours later, the Bravo Team of the Special Tactics And Rescue Service (S.T.A.R.S.) police force is sent to investigate a series of cannibalistic murders in the Arklay Mountains outside of Raccoon City. On the way to the scene, its helicopter has an engine failure and crash-lands in a forest. While searching the immediate area, Officer Rebecca Chambers of Bravo Team comes across the Express, now motionless. She begins to investigate the scene, only to find the passengers and crew transformed into zombies. She is unaware their transformation was a result of exposure to Umbrella's T-virus contained within the leeches. As she explores the train for answers, she teams up with Billy Coen—a former Marine Force Reconnaissance officer, who was to be executed for killing 23 people until the military police van transporting him crashed within the region.
The pair soon notices the same mysterious young man, moments before the train suddenly begins moving again. Unbeknownst to the pair, two soldiers from Umbrella, on the orders of Albert Wesker and William Birkin, attempt to take control of the train and destroy it, but are killed by leeches before they can complete their mission. As the train speeds out of control, Rebecca and Billy apply the brakes and avert its course towards an abandoned building. Upon exploring the area, they discover it to be a disused training facility for future executives of Umbrella. They also discover that the former director of the facility and the corporation's co-founder, Dr. James Marcus, was responsible for discovering the so-called Progenitor virus in the 1960s, and decided to examine its potential as a biological weapon. He combined it with leech DNA to develop the T-virus that causes rapid mutations in living organisms and thus transforms humans and animals into zombies and monsters.
As the pair continue to explore the facility, Wesker decides to leave Umbrella and join its rival company, and makes plans for further research on the T-virus. William Birkin refuses his offer to join him, instead opting to complete his research on the G-virus. Later, Rebecca becomes separated from Billy. On her own, she encounters Captain Enrico Marini, who tells her that the rest of the Bravo team will meet up at an old mansion they found, but allows her to stay behind to find Billy. Just after Enrico leaves, Rebecca is attacked by the Tyrant. After temporarily defeating the Tyrant, Rebecca meets up with Billy again and, together, they defeat it and continue on towards a water plant.
Eventually, Rebecca and Billy catch up with the leech-controlling man, who happens to be Marcus' final experiment, Queen Leech. Ten years earlier, Marcus was assassinated on the orders of Umbrella's other co-founder, Oswell E. Spencer, who sought his research. After his corpse was dumped, Queen Leech entered his body and reanimated it, gaining his memories and the ability to shapeshift, thus believing itself to be the resurrected Marcus and orchestrated the T-virus outbreak in the facility and on the train as a means of revenge against Umbrella. After temporarily defeating it, Billy and Rebecca attempt to escape to the surface via a lift, just as William Birkin trips the facility's self-destruct mechanism. Pursued by Queen Leech, the pair eventually kill it and escape before the facility is destroyed. Following their escape, Rebecca notices the mansion that Marini mentioned and prepares to head for it. Before she does, she assures Billy that her police report will list him as another casualty of the incident. Thanking her for his freedom, Billy departs as Rebecca heads towards the mansion to seek out the whereabouts of her fellow Bravo Team members (seen in Resident Evil).