Rachel's boyfriend Frank is different from other people. His strangeness is part of what she loves about him: his innocence, his intelligence, his passionate immersion in the music of JS Bach. As a coder, Frank sees patterns in everything, but as his theories slide further towards the irrational, Rachel becomes increasingly concerned for his wellbeing. There are people Frank knows online, people who share his view of the world and who insist he has a unique role to play. In spite of Rachel's fears for his safety, Frank is determined to meet them face to face. When Frank disappears, Rachel is forced to seek help in the form of Robin, a private detective who left the police force for reasons she will not reveal.