A lone female park ranger tries to track down a vicious creature killing various people and terrorizing her at a remote national park.

Police lieutenant detective Thomas 'Thom' Randall's steady girl-friend, Dr. Jennifer 'Jen' Allen, is Dr. Aaron Michaels's main assistant on his pharmaceutical firm Bellion's research program to cure the highly contagious, fatal infection Guinin. A convicted murderer, whom Thom arrested, is one of their special drug test subjects, but escapes. The convict and Jen are affected by radioactively altered DNA from an experimental reactor used on mosquitoes which transfer quinine. The convict soon mutates into a mosquito-like monster, which sucks its victims dry. By the time Thom and his junior murder brigade partner Charlie Morrison figure out what happens, Jen starts mutating herself.