The series chronicles the adventures of four turtles and their master and adoptive father, the rat Splinter, living in the sewers of New York. Exposure to a mutagen turned all five of them into human-like creatures. Trained by Splinter in the art of Ninjutsu, the four turtles fight against various threats to the city, be it other ninjas, mutants, aliens, criminals or sometimes even supernatural creatures, all trying to conceal their existence from the public.