The city of San Siroco is sinking. The basement of Dr. Tamsin Rivers, the arrogant, selfish head of the research team assigned to find the source of the subsidence, is sinking faster. As Tamsin grows obsessed with the distorting dimensions of the room at the bottom of the stairs, she finds a door that didn’t exist before --- and one night, it opens to reveal an exact physical copy of her. This doppelgänger appears fully, terribly human, passing every test Tamsin can devise. But the longer the double exists, the more Tamsin begins to forget pieces of her life, lose track of time and grow terrified of the outside world. As her employer grows increasingly suspicious, Tamsin must try to hold herself together long enough to figure out what her double wants from her, and just where the mysterious door leads.

Practical, unassuming Jane Shoringfield has decided that the most secure path forward is this: a husband, in a marriage of convenience, who will allow her to remain independent and occupied with meaningful work. Her first choice, the dashing but reclusive doctor Augustine Lawrence, agrees to her proposal with only one condition: that she must never visit Lindridge Hall, his crumbling family manor outside of town. Yet on their wedding night, an accident strands her at his door in a pitch-black rainstorm, and she finds him changed. Gone is the bold, courageous surgeon, and in his place is a terrified, paranoid man --- one who cannot tell reality from nightmare, and fears that Jane is an apparition that has come to haunt him.