Break Of Dark

Is there a barrier that divides the dark unknown from the everyday world around us?

If so, is it broken sometimes by the dead returning, by the undead, or by alien creatures?

What else could account for the chance meeting (or was it?) between a young student and hitch-hiker who turns out to be so much stranger than she seems? Why else should three successive crews flying a Second World War bomber - Blackham's Wimpey - be driven to madness, despair, even to death, though the plane returns fr...