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. Like Frank, Robin is obsessed with the music of Bach.
Like Frank, she has unexplained connections with the criminal underworld of southeast London.
An obscure science fiction story from the 1950s appears to offer clues to Frank's secret agenda, but not to where he is. As Robin and Rachel draw closer in their search for the truth, they are forced to ask themselves if Frank's obsession with an alien war, against all logic, might have a basis in fact.
