Resident Evil 0 was released exclusively for the Nintendo GameCube on November 12th, 2002 and is the 5th game in the series in terms of release date (counting the remake of the original to be a separate game) but is chronologically the first game in the series. It is also one of the last games in the main series to use the original control scheme & design, with Outbreak - File #2 being the very last.
It uses a team mechanic in which the player can control two characters in order to solve puzzles and defeat foes. It was originally slated to come out for the Nintendo 64 and is currently a Nintendo platform exclusive.
A Nintendo Wii version of the game was released in 2009 and features minimal changes, the main change being that the game can be controlled with the Wiimote.
The gameplay in Resident Evil 0 is very similar to that of GameCube remake of Resident Evil (which was released earlier in the year) but is the first and only game in the series to have the player take control of two characters simultaneously. Billy and Rebecca can be controlled separately or as a team. The separation element is included as the focus of many of Resident Evil 0's puzzles. For example, in one section, Billy controls a gondola while Rebecca (due to her smaller size) rides in it and into a new area.
Each character has their own unique abilities and traits that allows for them to be specifically useful at certain points in the game. Rebecca is the only character who can mix herbs and is smaller so she can fit into tight spaces. Billy can move heavy objects, take more damage and uses a lighter (which needs fuel).
"Item Zap"/Exchange System
An interesting feature that would be the first time used in the series was the integration of item exchange or "item zap" as it was code named. This was utilized as means of passing around items from each character, mostly to solve puzzles, although of course the player can also utilize this to transfer healing items, weapons, and ammunition. This system would become a series staple, most notably used in Resident Evil 5's co-op system.
Unique to the series is the disappearance of item boxes scattered around the game, replaced with the ability for players to be able to drop carried items and pick them back up exactly where they left them.
Resident Evil Zero contains a unique final boss sequence where Billy must protect Rebecca while she is opening four different valves. The player loses control of the switch system and can only attack using Billy, who cannot actually cause damage to the Queen Leech but distract it until the final valve is turned.
Much like previous instalments in the series, Resident Evil Zero features its own Extreme Battle/Mercenaries mode. The concept is that Billy and Rebecca must collect a certain amount of color-coded, leech-themed jewellery pieces before exiting out the Umbrella Research Facility front door. When collecting leeches it can only be done specifically for each character, blue for Billy and green for Rebecca, but they cannot swap using the exchange system or pick up a the opposite color for their team mate. The player also has the ability to select an alternate outfit for both characters.
Resident Evil 0 takes place on July 23rd, 1998. The S.T.A.R.S. Bravo team is dispatched to the Arklay Mountains to investigate and find the cause of the murders that have recently occurred. While over the forest the Bravo team's helicopter is mysteriously forced to crash land. Upon leaving the crash site, the Bravo team discovers an overturned Military Police vehicle, as well as orders for the transport of a prisoner to be executed.
Rookie team medic Rebecca Chambers discovers the Ecliptic Express stopped and without power in the middle of the forest. Upon entering the train, Rebecca encounters numerous zombies, and infected canine. Pressing further throughout the train, Rebecca meet Billy Coen, the prisoner that was being transported to his execution. With little control over the recent events, Rebecca reluctantly agrees to team up with Billy, to get to safer ground, still vowing to bring him into custody if they both make it out alive. Further exploring Billy and Rebecca meet what appears to be a possible survivor, but before getting too close they find out it is a humanoid leech. Billy disposes of the leech creature, afterwords they see a lone man outside of the train standing on a cliff who appears to have some sort of control over the leeches. Unable to identify the mysterious individual, they must find a way to restore power to the train and get out of the forest. Once, power is finally returned to the locomotive, the train speeds to an unknown destination and eventually derails.
Seemingly unscathed from the train crash Rebecca and Billy end up at the trains destination. The mansion like building, is actually revealed to be a training facility for new employees of the Umbrella Corporation, and upon entering they are welcomed with a pre-recorded message from the head of the facility, James Marcus. During their trek through the training facility the pair is face with numerous zombies as well as plenty of new abominations. Over the course of their time together, Rebecca begins to see that Billy isn't a quite the criminal type, and begins to wonder why he was sentenced to be executed. The two eventually discover an underground portion of the training facility and are separated when the floor that Rebecca is standing on collapses. After being rescued by Billy, she questions him about what he did to get the sentence he had been handed. Billy finally explains, that he was part of the USMC (United States Marine Corps) and during a mission in Africa, his commanding officer ordered them to exterminate civilians of an African village that was suffering from a civil war. Billy refused to follow the orders, and once the information proved to be false, the slaughter was blamed entirely on him.
Upon entering an abandoned sewage treatment plant, the duo encounter James Marcus. Marcus, had been monitoring Coen and Chamber's progress throughout the training facility as well as the other areas they visited. Marcus reveals that a decade ago Lord Ozwell E. Spencer has Marcus assassinated, in the midst of the research on his leech research. He explains that the Queen leech from his experiments crawled into his mouth after he was shot, and the T-Virus that the leech was injected with was able to resurrect Marcus. Midway through his speech, the Queen leech takes over Marcus' body, and causes him to mutate into a new type of leech monstrosity. After toppling the Queen leech, Rebecca and Billy escape the training facility just as it is rocked by a gigantic explosion. After the escape, Rebecca takes Billy's dog tags as "proof" that he didn't survive the incident, and then the pair amicably part ways. In the distance Rebecca is able to see the Spencer mansion...