Professor Brian Newman is researching into a cure for meningitis. Unfortunately, as a by-product, he accidentally creates a lethal mutated version of the disease which is swiftly killing all the bats in his glass test cage. He knows that his experiments have failed and plans to dispose of the bats as soon as all of them have finally succumbed.
But, fatally contaminated bats escape to spread their disease around the country. The only possible way to prevent an epidemic is to locate and destroy every single bat; but bats are small, they don't come out during the day and even a country as relatively small as UK has more than enough area to hide them.