Five years prior to the series, a massive explosion released high concentrations of nanites into the atmosphere, infecting every living thing on Earth. These nanites may randomly activate inside their hosts, mutating the subject into a monster known as an E.V.O. (or "Exponentially Variegated Organism") - usually a mindless creature that attacks everything around it, though some E.V.O.s retain human-level intelligence. To combat the E.V.O. threat, an international organization known as Providence was created.