At UNIT headquarters, the Brigadier and the Third Doctor are discussing the virtues of pop music. The Brigadier dismisses the modern music to which the Doctor is listening. He expresses a preference for Bing Crosby, Frank Sinatra and Vera Lynn. The Doctor, however, chides the Brigadier for not examining the effect of music on the youth of Britain.
Meanwhile, in the middle of the English countryside, a musical threesome, the Swifts, have become stranded after a gig. The sky opens up on them, and they begin to wonder why they've bothered with the "pop scene" at all. Out of nowhere, a Rolls-Royce shows up and a mysterious man inside begins to have an hypnotic effect on them.
The next day, at the most popular radio station in Britain, disc jockey Billy "The Kid" Kiddsley is in the midst of his top-rated show when suddenly he can't think of anything to say. His mind is searching desperately for something, anything, when he remembers a man who had stopped him on the way to work, a man with deep, deep eyes who had given him a new record to play.
As if on auto-pilot, Billy drops the needle on the new vinyl. Out comes the music of a new group, the Four Horsemen — made up of the three members of the Swifts and a new member — the Master. The music instantly affects everyone listening to it. It compels ten million people across Britain to begin converging on Stonehenge.
It's a scientific fact that certain combinations of notes, certain harmonies, can be acutely painful — can cause near-madness. Just think of the use to which a skilled scientist could put that knowledge!
Back at UNIT headquarters, the Brigadier cannot help but notice the changed traffic patterns. Every organ of government is terrified by the mass migration. At the Doctor's suggestion, he orders the power to the radio station cut, but it is too late. The Master's instructions have already been implanted in the minds of the millions that have heard the "music" of the Four Horsemen. As the Doctor points out, however, the problem is that no one knows what instructions the Master has given. If the millions have been instructed to destroy Britain, then Her Majesty's government faces a stark choice. They may be forced to drop a nuclear weapon on Stonehenge to protect the rest of the country.
While the military mobilises, scrambling fighters to a defensive position over Stonehenge, the Brigadier and the Doctor hop a helicopter to Salisbury Plain. They discover the Master has set up his own, independently-powered sound system. It continues to keep his "troops" under his control. When the Brigadier tries to rush the Master, the evil Time Lord captures Lethbridge-Stewart with his machine.
Now mentally possessed by the Master, the Brigadier turns on the Doctor. He savagely attacks his scientific advisor, felling the Doctor to the ground. Believing the Doctor dead, the Brigadier attacks the next-closest target — the Master. The Master slips away, but not before the Brigadier destroys the mind control machine with his bare hands. Once this is done, the Brigadier naturally comes to his senses.
Simultaneously, the Doctor rises, revealing that the Brigadier's attack wasn't that bad — at least, not for a Time Lord's physiology. The Brigadier's still foggy, though, and asks the Doctor to explain "what the blazes" happened. The Doctor reveals that the Master made a critical error with his murderous mind control. He told the Brigadier to "kill", without being any more specific than that. As a result, the command backfired, when the Brigadier caught sight of the Master.
The two friends notice that the crowd of three million people also seem to be waking up. The Doctor suggests that they get airborne in the helicopter to begin directing the effort to get all the people back home.
Before they do, though, they meet up with one of the members of the Swifts. He explains that he doesn't know how he got here, and that the last thing he remembers is that he was about to give up on the pop scene. The Doctor considers his statement and tells the boy that getting out of pop music might be a very wise idea.
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