It's a continuation of the Parasite Eve series, which hasn't had a sequel since the PS1 era. The basic gist of it is that human mitochondria (a real organelle within animal cells) has evolved from a parasitic single celled organism and is now destroying humans and animals by mutating them from the inside out. The series follows Aya Brea, a New York cop who is the sole survivor of the first game, and in the second she joins a government special forces team called MIST that investigates mitochondrial mutants. I haven't played The Third Birthday yet, but I'm assuming it picks up some time after Parasite Eve 2.