On June 18, Churchill addresses the House of Commons about France’s defeat and warns that the Battle of Britain is about to begin. He declares that Britain now stands between Hitler and a wider descent into a "new dark age," ending with the line that this could be Britain’s "finest hour." That night he rereads the speech on radio, but the broadcast lands poorly because his delivery sounds strained; some listeners even think he is drunk, though the problem is largely mechanical because Churchill insists on speaking with a cigar in his mouth.

The next day, Britain’s chiefs of staff send Churchill and the War Cabinet a secret warning that invasion must be treated as an immediate threat. They predict that an air assault will come first and that the next three months may decide the war. As German raids intensify and public anxiety rises, Churchill focuses on the possibility that the Luftwaffe is using secret radio beams to guide night bombing accurately, which would make Britain far more vulnerable.

On June 21, officials gather in the Cabinet Room at No. 10 for a highly secret meeting on beam navigation. R. V. Jones, the young scientist whose work prompted the meeting, is summoned late and arrives after it has begun. Faced with Churchill, Lindemann, Beaverbrook, Tizard, Sinclair, Dowding, and others, Jones explains the evidence like a detective story and presents new intelligence suggesting that the German Knickebein system uses intersecting beams on known frequencies to lead bombers to targets.

Churchill quickly grasps the danger: if German bombers can navigate precisely at night and in bad weather, they can attack without the normal limits imposed by daylight fighting and fighter escorts. Jones proposes immediate reconnaissance flights to find the beams and then develop countermeasures such as jamming or spoofing them. Though Tizard remains skeptical and hostile, Churchill accepts Jones’s argument, orders the search to begin at once, gives beam countermeasures top priority, and again vents frustration at bureaucratic delay while pushing Lindemann’s other defensive ideas, including aerial mines. Tizard, angered at being overruled, resigns soon afterward.

The same day also brings Churchill private relief: Brendan Bracken, using money from Sir Henry Strakosch, deposits £5,000 into Churchill’s bank account and averts an immediate personal financial embarrassment. The next day, Flight Lieutenant H. E. Bufton reports that his reconnaissance flight has found the expected narrow beam, matched one of the predicted frequencies, and detected a second beam crossing near Derby, apparently marking the Rolls-Royce Merlin engine factory as a target. Jones’s theory is confirmed, the beam system is code-named Headache and the countermeasures Aspirin, and after the breakthrough Jones and a colleague celebrate by getting drunk.