In the aftermath of the Martian invasion that nearly brought Earth to its knees, a new threat looms in the shadows. The Martians, defeated by Earth's tenacious microorganisms, have been vanquished, but the danger remains. Unseen conspirators from Earth aided the invaders and now plot to send bacterial samples to Mars. Their nefarious aim is to assist the Martians to conquer the microscopic foes and then return to Earth as unstoppable conquerors...