Scottsville, a little quiet city of the United States. Professor Arthur Fuller undertakes researches on bats in the science laboratory of the university. Several hundred of them are enclosed in cages. They seem abnormally aggressive and one clutches to the long blond hair of one of the two students who came to feed them. The same evening, the shredded body of the professor Fuller is found in his laboratory. All the bats are flights. On places of the drama, John Winslow, the veterinarian of the city, a forty of years widower, meets Ally Parks in charge of the inquiry.
Scottsville is also in prey of a rich unscrupulous constructor, Carl Hart, who does not hesitate to expropriate residents of a little valley to construct a plot that will bring him substantial incomes. Hart sees a bad eye the arrival of the detective Ally Parks who privileges the thesis of the murder rather than the accident. The event risks to make a bad advertisement for the city and his enterprise.
Again, the bats in liberty attack. Having found refuge in habitations in construction of the future plot of Hart, they throw on a security guard who finds death in atrocious conditions. These small animals seem to attack just on order of a little electronic device set near the future victim, simulating them meal time. Winslow recuperates one of the specimen that the guard killed before to die. A thorough study reveals that murderess bats have been genetically modified to be transformed into real killers. Ahead the danger that threatens, Ally attempts to convince her superior, the Police Chief Sam Taylor, to cancel the expected festivity evening by Carl Hart for the promotion of his plot. The former, paid as it must be by the constructor, refuses categorically and obliges Ally to stop her inquiry.
Ally and John nevertheless go to the party. What was foreseeable produces. A furious bats cloud throw on guests. There again, a small ultrasonic device remote commanded by a man remained in the shade has released the hostility of the little carnivorous rodents. Ally Park and John Winslow repel the beasts with the help of aluminium pieces that disturb their sonars. Carl Hart is furious, for him it is bankruptcy.
Sam Taylor is the next victim of the cloud of vampires. This time Ally has understood that attacks are remoted controlled. John supplied with an ultra high frequency detector notices the source of a new emissions of the murderer...
Spoiler ! the end of the story is revealed below…
The new signal emanates from the Kramer's farm. Precipitating there, the two inquirers discover Hart tied on a chair, a high receiver frequencies on his chest, hundred of bats patiently waiting for diner time. Both equally discover who is behind this plot. It concerns the professor Fuller who explains that the valley belonged to his father, hunted as a dirty by Hart and his money thirst. His revenge desire pushed him to kill every people responsible for his death. Ally and John liberate Hart in extremis and make explode the farm with their residents in: bats and their creator; the professor Fuller.