When Rachel Carson published Silent Spring... despite the response, maybe people should have listened a little more. But now it's too late. The planet has become overwhelmingly polluted, mutating and turning evil all of the creatures who walk on the face of the earth. In order to save the human race, you have been dispatched to completely destroy all within your sight, so that the earth may hopefully once again start anew. Otherwise, you'll find even your home base over run with toxic creatures that will restrict your breathing and give you severely harmful diseases. Now go fourth and eradicate.