When the first letter appears in Kay Anderson's locker, it carries one instruction: dye your hand blue.

She definitely shouldn't follow it. Kay doesn't have time for secret societies: as a scholarship student at up-and-coming Manhattan prep school Davison High, she knows her job is to work hard, get into Northwestern, and ignore her wealthy classmates' fun-filled Instagram stories. Besides, her first and only real friend at Davison died suddenly last year.

Still, Kay's intrigued, so she stains her palms with ink to join the mysterious Blue Hand Girls, sharing an unspoken thrill with the classmates who do the same. More letters show up, assigning risky initiation tasks, and Kay realizes the group is set on exposing the shady business that Davison's founders would rather keep hidden―things that her dead friend might have known about.

But the anonymous instructions also demand the girls reveal their own secrets, bonding them all too close to abandon the society. Soon Kay doesn't know who's more dangerous: the powerful people who run her school, or the Blue Hand Girls themselves. And then there's Zola Wolfe, the beautiful redhead in Kay's Calculus class, who's never been seen with a blue hand, and who just might become her girlfriend. Yet Kay can't help but wonder, even as she kisses her on the roof of Davison High, if Zola is the most dangerous of them all.