In 1979, a teenage girl purchased a pair of American alligators while on vacation with her family at a tourist trap in Florida. When the family returns home to Chicago, the girl's surly, animal-phobic father Mr. Nakamoto promptly flushed it down the toilet and into the city's sewers.

The alligators survived by feeding on the discarded carcasses of apes used in illegal experimentation and dumped into the sewers. These animals had been used as test subjects for an experimental growth formula intended to increase agricultural livestock meat production. However, the project was abandoned because while the formula had the desired effect of making the animals larger than normal, it had the unwanted side effect of massively increasing the animals' metabolism, causing them to develop an insatiable appetite. 

During the 43 years spanning 1979 and 2023, the baby alligators bioaccumulated concentrated amounts of this formula from feeding on these carcasses, causing it to mutate and grow into a perpetually ravenous 36 foot (11 m) monster that can not only walk as humans, but created a extremely intelligent specimen with the ability to communicate with all animals and humans. 

Today these creatures decided to leave the sewers and make their home in the alleys of the the Windy City.