Following the cover up of a crash of a nuclear bomber at an American Air Force base in the UK, Dennis Markham, a prominent Member of Parliament and opponent of the American nuclear presence in the United Kingdom, is planning to ask questions about it in Parliament. Before he can, he is reported by a London paper to have been seen leaving a woman's home on the same evening as she is visited by a military attaché from East Germany, Markham's loyalty to his country is questioned. He is hounded by the media and is forced to resign. The author of the newspaper exposé, Nick Mullen, continues his work alongside colleague Vernon Bayliss who suspects that Markham was framed. When Bayliss dies from a supposed heart attack the same night as Bayliss' flat is ransacked by someone who was not after money or valuables, Mullen suspects something deeper at work. He then finds some evidence of the cover-up of a near-accident at a nuclear site and a secret US Air Force base. With the help of Markham's secretary, Nina Beckman, Mullen continues to investigate the affair despite a break in of his flat, surveillance and other attempts of the British Government to stop him. In the end, Mullen and Beckman are seemingly killed in an explosion, but Mullen's story about the cover-up is published.
Halliwell's Film Guide described it as an "efficient political melodrama, basically too old-fashioned to start a cult." Denholm Elliott's performance has been singled out for particular praise. Roger Ebert wrote: "The acting is strong throughout, but Elliott is especially effective. What is it about this actor, who has been in so many different kinds of movies and seems to make each role special? Here he is needed to suggest integrity and scruples, and does it almost simply by the way he looks... Defence of the Realm ends on a bleak and cynical note – unless you count the somewhat contrived epilogue – and gets there with intelligence and a sharp, bitter edge." Radio Times gives the film four stars out of five, claiming: "The role of the sozzled veteran reporter who for once finds himself involved in a meaningful story is brought wonderfully to life by Denholm Elliott... Gabriel Byrne, as Elliott's ambitious young colleague, is less effective, but the film has plenty of tension and co-star Greta Scacchi proves a worthy accomplice."
