What happens when you suddenly become aware of a stranger walking around inside your head, asking questions to which you've known the answers all your life?
What happens when you realise, slowly but surely, that the telepathic visitor is an alien, and that the purpose of his visit is invasion from another world ... and a very unpleasant invasion, at that: one that requires the elimination of half the human race by robots.
It's one thing to know what will happen, but another thing entirely to convince everybody else of the truth.
The only people who believed John English were his own family, who were themselves being prepared as hosts for the invasion commanders.