In July 24, 1998, when a series of bizarre murders occur on the outskirts of the Midwestern town of Raccoon City, the Raccoon City Police Department's S.T.A.R.S. team are assigned to investigate. After contact with Bravo Team is lost, Alpha Team is sent to investigate their disappearance. Alpha Team locates Bravo Team's crashed helicopter and land at the site, where they are suddenly attacked by a pack of monstrous dogs, killing one of the team's members. After Alpha Team's helicopter pilot, Brad Vickers, panics and takes off alone, the remaining members of the team (Chris Redfield, Jill Valentine, Albert Wesker and Barry Burton) are forced to seek refuge in a nearby abandoned mansion.
Depending on which character the player assumes control of, either Chris or Barry are separated from the rest of the team during the chase and do not make it to the mansion. At this point, the team decides to split up to investigate. Over the course of the game, the player character may encounter several members of Bravo Team, including Enrico Marini, the captain of the S.T.A.R.S. Bravo Team, who reveals that one of Alpha Team's members is a traitor before being shot and killed by an unseen assailant. The player character eventually learns that a series of illegal experiments were being undertaken by a clandestine research team under the authority and supervision of biomedical company Umbrella Corporation. The creatures roaming the mansion and its surrounding areas are the results of these experiments, which have exposed the mansion's personnel and various animals and insects to a highly contagious and mutagenic biological agent known as the T-virus.
Eventually, the player character discovers a secret underground laboratory containing Umbrella's experiments. In the lab, the player encounters Wesker, who reveals that he is a double agent working for Umbrella, and plans to use the Tyrant, a giant humanoid supersoldier, to kill the remaining S.T.A.R.S. members. However, in the ensuing confrontation, Wesker is supposedly killed and the player character defeats the Tyrant. After activating the lab's self-destruct system, the player character reaches the heliport and manages to contact Brad for extraction, at which point the player may be confronted by Tyrant one last time. The game features multiple endings depending on the player's actions at key points over the course of the game. In the best ending, the surviving team members escape by helicopter as the mansion self-destructs. In contrast, in the worst possible ending, the mansion remains intact and the player character is the sole survivor.