On a sunny morning in June, Margaret Carpenter wakes up to find a new iPhone on her doorstep. She switches it on to find a text from her best friend, Charity Atwater. The problem is, Charity's been missing for over a month. Most people in town--even the police--think she's dead.

Margaret and Charity have been lifelong friends. They share everything, know the most intimate details about one another...but Charity carries a secret that even she is unaware of. A secret engraved into her DNA helix. For Charity is also known Subject Six, the crown jewel of Project Athena--a clandestine and unorthodox gene manipulation experiment, the brainchild of tech titan Rudyard Crate. And when Charity's gene sequencing actualizes during a traumatic event at a high school party, it sets in motion a chain of events that will end in tragedy, bloodshed, and death.

And now Charity wants Margaret to know her story--the real story. In a narrative that takes place over one feverish day, Margaret follows a series of increasingly dreadful breadcrumbs as she forges deeper into the mystery of her best friend--a person she never truly knew at all...