They’re like mirages, illusions, things not quite there, ripples in the fabric of time and space. The people of earth call them ghosts, believe them to be their dear, departed relatives and welcome them into their homes, but the Doctor senses they’re not what they seem.
People see what they want to see. The figures are vaguely humanoid; it’s easy to imagine a resemblance to parents, grandparents, friends. But belief doesn’t create fact or alter truth.
When the eerie figures finally step through the veil, they’re not ghosts, they’re something far more deadly.
And it’s too late to stop them.