""There's no turning back now, but it'll all be worth it.""
— Monica executes her colleague and steals the G-Larva.
In late 1998, as the t-Virus ravaged both the city above and below ground, Monica attempted to acquire a G-Larva. Being a ruthless and mercenary individual, she murdered a fellow researcher in order to procure the larva, presumably intending to sell it to the highest bidder.
Her escape was hampered when the train she had intended to use (which would later be used by Leon S. Kennedy, Claire Redfield and Sherry Birkin during their own escape a few days later) would not function. After abandoning hope of escaping on said train, Monica encountered her former colleague; an amnesia-stricken Yoko Suzuki. Monica stole Yoko's Umbrella Corporation ID card at gunpoint before taunting her and alluding vaguely to her unknown past. She then left Yoko, intent on finding another escape route.
Using Yoko's ID card to access another possible escape point out of the facility, Monica attempted to use the lab's disguised train yard turntable back entrance/exit lift to escape to the surface. Her efforts came to nothing, however, as the lift was frozen into place by the facility's climate control system having being set to below freezing point by another researcher, Frost, in order to slow the creatures that had infested the facility. Immediately after, Monica was attacked by one of those creatures.
Monica then reappeared as Yoko and two other survivors were about to use the lift to exit the facility after restoring the climate control to a temperate level, hence making the lift operational once again. By this time, Monica had become the host of the very G-Larvae she had stolen and attempted to escape with; she falls to the floor, unconscious. Despite her betrayal, Yoko pitied Monica, asking "why did this have to happen?" and leaving her to her fate. As the lift ascended, the larvae burst from Monica's chest, instantly killing her, and made its way up the shaft to attack Yoko and the survivors as they ascended to the surface.