The series features a diverse set of five characters who live in different parts of England, all of whom have been blackmailed by the same individual (referred to in the credits for episode seven as "Black Gloved Man"), who has given them each a letter with the message "I know what you did…" In the second episode, the blackmailer leaves them a second message that reads, "You killed her". In the third episode they receive a videotape showing them in an asylum together (several having previously revealed that they had been institutionalised) performing "Close Every Door" from the musical Joseph and the Amazing Technicolor Dreamcoat. It is later disclosed that the institution was called Ravenhill Hospital. In Episodes Five and Six, the characters discover the final message: a key depicting a raven. At least David's letter also contains the message "I'm waiting...".
