Summary: RAF film giving a brief look at the UK's air defence organisation. Beginning with Battle of Britain scenes of plotting rooms and Spitfires being serviced and then taking off, the film goes on to look at a present day air defence exercise. Unidentified aircraft are detected approaching the UK from eastern Europe, and the crews of Lightning and Javelin fighters increase their alert state accordingly. The aircraft then take off and are shown flying in formation before returning to land at their home airfields. New crews take over the alert and Lightnings take off at night as the narrator emphasises the importance of constant vigilance.
