On a cold morning in central London, Doctor Meredith Shaw awakens on what soon becomes her worst nightmare come to life. When a young woman is discovered murdered in an alley, Shaw is called in to confirm the MO of a serial killer only known as 'Hummingbird', who has been in hiding for 6 years. With the wary realisation that Hummingbird has returned weighing on their minds, Shaw and her associate DCI Leon Burke embark on a case lead by the dead ends and puzzles left behind from the old files. With past grievances driving Shaw to her limits, her tactics of playing into Hummingbird's hand seem a dangerous attempt to beat him at his own game. But after a year of terror and a sudden six year absence, Shaw is determined to find out why he has come back at all, and to finally discover the truth about Hummingbird's identity. Deeming her the only one up to the challenge, Hummingbird tasks Shaw not only with solving the ultimate puzzle, but what place she holds in his ever-changing game.