A dinner party is taking place in the London home of Jack and Claire Trent. Their three guests are a Mrs Violet Eversleigh, Sir Alington West (a noted psychiatrist), and West's nephew, Dermot West. The talk turns round to precognitive abilities and premonitions, of which Sir Alington is dismissive, believing them to be both coincidences and situations that are talked up after the event. Dermot is not so sure, describing such feelings as having a red signal – "danger ahead!" – and tells one story of near-death in Mesopotamia, when he avoided being murdered by an Arab servant. What he does not tell the group is that he is again experiencing the red signal tonight at the dinner party. He ponders who could be the source of danger, his thoughts moving to his love for Claire Trent, a feeling that he constantly suppresses in view of the fact that Jack Trent is his best friend. Coming out of his reverie, he finds the conversation has turned to madness and the dangers of suppressing delusions. Sir Alington looks pointedly at Claire Trent, who is visibly disturbed by this talk.