Samantha McAllister looks just like the rest of the popular girls in her junior class. But hidden beneath the straightened hair and expertly applied makeup is a secret that her friends would never understand: Sam has Purely-Obsessional OCD and is consumed by a stream of dark thoughts and worries that she can't turn off.

Second-guessing every move, thought, and word makes daily life a struggle, and it doesn't help that her lifelong friends will...