Clover and Herbie are aliens who came to Earth when their planet was destroyed. They planted mind-control devices in government officials. They are trying to revive their dying race by having the crabs carry fertilization material in their bites. After four months of a bite, the victim explodes into baby Clovers. This experiment is going on all over the world.

Due to the stress of interstellar travel the females went infertile. The scientists working for the government recently figured out how to fix that so they put a male of the species into the city to fertilize Clover. Either that, or Clover is actually male and Herbie is female. Whichever.