Sometime between the late night and early morning hours, in a dank, uncomfortable interrogation room, sits a young woman about to tell you a chilling story. Her pregnant sister, Sarah, is missing. Her cousin is somehow involved. They were all down by the Schuylkill River last night, she says, but she won't say why. She's cold, terrified, and confused about what happened. Or is she just lying?

You want to believe her. Her story seems plausible on the surface. But as you dig deeper, it's clear something is very wrong. Instead of helping to find her sister, she keeps telling you stories about her ex-roommate, Gemma. Gemma, who came into their lives offering a six-month cash deposit up front, proffered from a dirty gym bag. Gemma, who disrupted their routines with her manipulative behavior, her bizarre habits, and her secrets. Gemma, who disappeared quietly several months ago, leaving no trace of where she went or that she had ever lived with them all.

Who was Gemma and what happened to her? What does it have to do with Sarah's disappearance? Are they part of some elaborate game or is something else going on? And what happened by the river last night? Why can't she just tell the truth?

From the author of They Did Bad Things, Gemma is a devious new mystery and slowly escalating psychological thriller with a touch of horror.