Brooklyn, 1942. War rages overseas as brother and sister Will and Maggie Scripps contribute to the war effort stateside. Ambitious Will secretly scouts for the Manhattan Project while grief-stricken Maggie works at the Navy Yard, writing letters to her dead mother between shifts.

But the siblings’ quiet lives change when they discover a beautiful woman hiding under their back stairs. This stranger harbours an obsession with poisons, an affection for fine things, and a singular talent for killing small creatures. As she draws Will and Maggie deeper into her mysterious past, they both begin to suspect she’s quite dangerous - all while falling helplessly under her spell.

With whispers of spies in dark corners and the world’s first atomic bomb in the works, the visitor’s sudden presence in Maggie’s and Will’s lives raises questions about who she is and what she wants. Is this mysterious woman someone they can trust - or a threat to everything they hold dear?