She should have been safe...But she never came home.

Kath has lived on the same London estate for decades. Lately she's become friendly with the little girl next door, who is often left alone for hours at a time while her mum is at work.

They have a system. When Mina gets home from school to the empty flat, she knocks on the wall three times. Knock, knock, knock. I'm home safe and sound.

But one day Mina's knock doesn't come. Kath raises the alarm and the police begin the search. Kath was the only person looking out for Mina - now she might be the only one who can bring her home.

Unputdownable psychological suspense for fans of K.L. Slater, Shalini Boland and Louise Jensen.