Please note, due to the multiple scenarios it is nearly impossible to compile which incidents happened and did not happen during the first Resident Evil game. Discrepancies are to be expected.
In 1963 Ozwell Spencer, one of the founders of Umbrella, hired a man named George Trevor to construct a mansion in the Arklay Mountains. George Trevor was a famous architect with a wife named Jessica, and a daughter named Lisa. The mansion was designed with hundreds of mazes, secret passages, and puzzles, which only Spencer and Trevor were aware of.
On November 10, 1967, just a few days after the completion of the mansion, Spencer insisted that Jessica and Lisa visit the mansion. Meanwhile, Trevor was in New York City on a business trip. Jessica and Lisa became test subjects to the Mother Virus, a virus capable of amazing mutation. Three days later, Trevor arrived back from New York. Spencer told the man his family left to go visit their ailing Aunt Emma.
The next day, Trevor and Spencer took a tour around the mansion, and it was then that Spencer revealed he wanted to create Umbrella, a pharmaceutical company. Spencer made up a lie that the mansion would be used as a 'weekend retreat' for the workers, when really it was designed for research. The design of a mansion was chosen so no one would suspect the activities inside.
Trevor stayed in the mansion for a few days, but eventually began to worry about his family. He tried to find a telephone, but noticed a tunnel in the mansion he did not originally design. Soon, Trevor attempted to leave but guards took the man into an art gallery, telling him his family was dead. They then injected him with a sedative and locked him a room.
Days later, the man escaped from his room but couldn't find a way out of the mansion. Insanity had obviously gotten to Trevor, and he was unable to recall the solutions to the puzzles and mazes. It took him two days to remember where a secret passage was located. He arrived in a small tunnel with a tombstone reading his own name. He died shortly after.
In the mid-seventies, the Umbrella Corporation became a growing organization, and a training facility was built not far from the mansion. In 1978 Albert Wesker and William Birkin arrived as employees for Umbrella. After going through training they began research in the mansion on both the T-Virus and the mutated Lisa Trevor.
After years of research, Wesker and Birkin's studies led to the discovery of the G-Virus. Birkin was given the position of top researcher for the virus, and a new underground facility was created. Wesker was accepted a transfer, and the mansion was no longer occupied by the two men.
Not much happened involving the mansion for several years.
On May 11, 1998 an accident occurred and the T-Virus was spread. The present animal keeper of the facilities was ordered to wear a biohazard suit, while a lunatic scientist destroyed a water tank, causing part of the labs to become flooded. Only a day later, the keeper became irritated in the suit, while his mind became senseless, forgetting to feed the animals. After a trip to the infirmary, the keeper was told he no longer needed the suit. He tried to leave the mansion to visit his girlfriend, but was prevented. Meanwhile, the flooding facility underneath the dormitories created Plant 42.
Slowly, the keeper began to mutate and eventually ate one of the guards. Combining the outbreak on May 11th and the keeper's mutation, the virus quickly began to spread throughout the Arklay Mountains. The viral outbreak became worse as weeks went by, causing John, the chief researcher in the Arklay Labs, to write his goodbye letter to Ada Wong, his present girlfriend.
After numerous dead bodies were found in the Arklay Mountains, R.P.D. officials blocked off the entrance to the forest. On July 22, 1998 Albert Wesker - now the captain of the STARS team - was ordered to send his teams out to the mountains and lure them into the mansion. He was supposed to have them fight off the BOWs to collect combat data, as well as two embryos from each monster. The Bravo Team was sent to the Arklay forest to locate lost hikers. After losing contact with the Bravo team, the Alpha team was sent out, and Albert Wesker's plans were ready for action.
The Alpha team searched for the missing Bravo team, and discovered their crashed helicopter. The team was attacked by mutated dogs and forced to shelter in the mansion after their helicopter pilot, Brad Vickers, basically left them to fen for themselves. Wesker, not leading on to his ulterior motives, ordered Jill to explore the dining room after they heard a gunshot. Barry Burton chose to go with Miss Valentine. The two found themselves encountering a zombie. When they attempted to give their captain the horrifying news, they realized Wesker disappeared. Jill and Barry were forced to fight their way through the mansion in an attempt to find their teammates and discover exactly what went on in the mansion.
Chris, meanwhile, was caught in the same exact mess, but he soon ran into the one of the only surviving members of the Bravo team, Rebecca Chambers. They helped each other out during the entire mission, as did Barry and Jill. Each person found numerous files revealing what the mansion's true purpose was, and what Umbrella was up to.
Later Jill started suspecting Barry had some part in what was going on with the 'mission.' Several times he had dismissed her presence and questions, disappeared mysteriously, and even out-right said questionable things. He actually did have something to do with everything; however he was just "the Judas" blackmailed by Wesker. Wesker had Barry's family kidnapped in order for Barry to lead Jill and Chris to the correct places in the mansion, most importantly the labs, where Wesker planned to unleash the Tyrant. In advance, Wesker injected himself with a virus (obtained by William Birkin) to make himself a humanoid Tyrant. He let the Tyrant attack him so it seemed like he was killed.Jill, Chris, Barry, and Rebecca successfully be-ridded of the Tyrant while Wesker made his way to the self-destruct system. He set it off, causing the last few minutes in the mansion labs to be very suspenseful for the STARS team. They made their way up to the helipad where Brad Vickers safely flew them away from the nightmare.
As the team flew off they watched as the mansion blew to bits.