A man is seen fishing by a river, using fishing flies as bait. The narrator says that although these flies are a benefit to the fisherman, to most people and animals, flies are a menace. A horse is seen shaking off flies. A rubbish heap and a domestic kitchen are shown, both crawling with flies. A butcher's and a canning factory are also shown, with the narrator explaining that flies are a hazard in all these environments. Four fly specimens are shown: house fly, lesser house fly, stable fly and greenbottle fly. Breeding grounds are shown: a refuse tip and a piggery. The life cycle of a fly is shown, from larvae hatching from eggs to the flies hatching from pupae. A refuse tip with many fly pupae is seen. Flies are shown walking on a window, and their foot pads are shown. A close-up of a fly's head is also shown. The narrator explains that flies are a potential carrier of disease. In a laboratory, a scientist lets a fly walk over a petri dish. The resulting bacteria are cultured, and a path is clearly seen. Examples of bacteria spread by flies are shown, including typhoid and dysentery. Control measures are discussed. A man sprays a rubbish tip with insecticide. Rubbish collectors are seen, along with examples of a good rubbish bin and an ineffectual rubbish bin.