The SS Asia star is blown up in the Port of London killing many innocent seamen. The local Police begin to investigate but when Commander Brennan (Liam Redmond) of Special Branch becomes involved the investigation takes a more sinister path. Brennan believes the SS Asia Star explosion is sabotage planned by communist agents. But how do the saboteurs gain their information and what is their strategy? Brennan tracks down an enemy agent working for the Ministry of Supply who is passing information to the saboteurs via a contact at a musical society. Another saboteur, Jimmy Ellis (Kenneth Griffith), an idealist and former RAF pilot, has a change of heart and is kidnapped by the plotters. His mother (Joan Hickson) informs the Police but Brennan is already on the trail. The saboteurs next target are power stations with the aim of bringing the industrial output of the country to a standstill, but will Brennan and his agents uncover the plot and foil the communists in time?