Caithness, October 1871.

The Ordnance Survey are charting Scotland's most remote north-easterly county, a bleak landscape of endless moorland and lonely crofts. When a strange vision leads cartographer Robert Sutherland out onto the moor, an accident leaves him inches from death. He is taken to Leask House, to recuperate under the care of Mrs Sinclair and her beautiful daughter Isabel.

At first, Robert thinks the dreadful visions that plague him at Leask House are the result of the laudanum he has been prescribed. But as events take ever stranger and more terrifying turns, Robert begins to wonder whether his presence at Leask House is really a coincidence at all.

Someone - or something - has summoned him here.

And they don't intend for him to leave.