''Mum, there''s some people here from college, they asked me back to theirs. Just for an hour or so. Is that OK?''Midsummer 2017: teenage mum Tallulah heads out on a date, leaving her baby son at home with her mother, Kim.At 11 p.m. she sends her mum a text message. At 4.30 a.m. Kim awakens to discover that Tallulah has not come home.Friends tell her that Tallulah was last seen heading to a pool party at a house in the woods nearby called Dark Place.Tallulah never returns.2018: walking in the woods behind the boarding school where her boyfriend has just started as a head teacher, Sophie sees a sign nailed to a fence.A sign that says: DIG HERE . . .