Description: A film made by the Public Health Department of Hackney Borough Council illustrating air raid precautions services training and practice at the Home Office civilian anti-gas school and in the borough of Hackney. A biplane in flight drops a gas bomb into a rural field, where the effect of wind on its movement and distribution is observed. Onscreen text describes the difference between non-persistent gases, which move with the wind, and persistent gases, which spatter liquid about the point they burst. The hole made by the bomb and gas contamination of surrounding grassland is observed. Cards marked with chemical paint are used as indicators of the presence of mustard gas. Two topes of aircraft bomb are illustrated with cross sections of disarmed devices: the high explosive and the gas bomb. Fragments of a discharged gas bomb are also shown. A Gas-Mask Drill by members of civil air raid precautions services illustrates the use the Service Respirator and the importance of preparation for emergencies. A further drill shows officers of the Metropolitan Police using the Service Respirator. In a field a man equipped with a respirator is exposed to gas released from a canister. A series of men put on their respirators and enter a gas filled chamber within a building. Another gas chamber in the rear of a lorry is shown with its portable air lock entrance. In the field again, there is a test of poison gas on some sheds, with unsealed window and doors. The existence of gas is indicated by a chemical effect on two targets. A test is also made on an air lock entrance to a building. The air lock has been improvised from boards, battens and blankets. Some men in rubber uniforms present protective clothing for decontamination personnel and their dressers. These men then enact the treatment for contamination of grassland by mustard gas: a contaminated area is roped off, posted with danger notices and bleaching powder (chloride of lime) distributed. The danger of fire from using undiluted chloride of lime on mustard gas is presented in a small trial. There follows some scenes of training in decontamination work at the Home Office Civilian Anti-Gas School.