New York advertising executive and lifelong atheist Sarah Sheppard is highly successful, in line for a partnership, and feeling on top of the world. When she's visited by a mysterious client who offers her a job to write and market a comprehensive addition to the world's religious texts, she thinks it's an elaborate joke and turns him down. But God works in mysterious ways and she quickly finds she has no choice but to take the assignment.

Isolated at a remote estate in upstate New York, Sarah joins a group of scholars and theologians to compile The Addendum, but soon discovers that nothing and no one are what they appear to be. As more questions than answers mount up, Sarah has to decide whether to deny her natural skepticism or embrace that illusive idea of faith before she's nudged onto a path of no return.