The Sanctuary meets Shutter Island in this chilling speculative thriller from the acclaimed author of The Sisterhood.

BIRDS OF A FEATHER FLOCK TOGETHER.
SOME FLY TO THEIR DEATHS.

A group of students step off a platform into the path of an oncoming train.

Only Billie survives.

She remembers nothing.
But she knows they weren't suicidal.

At the secretive Arbor Institute, Billie is diagnosed with 'flock phenomena': a terrifying impulse to self-destruct with the group.

If it happens again, she'll die.

Cut off from the world, watched at every turn, Billie begins to understand the truth:

The Institute isn't trying to cure her.

It's trying to contain her.

But Billie isn't prey.

She's the threat.

'Brilliantly executed, alarming, deeply disturbing and worryingly believable!' Trevor Wood, author of The Inside Man

'Thoughtful and compelling... Moving and memorable' Jo Furniss, author of Dead Mile