In the grips of psychosis, a young man named Billy Knight (Joseph Quinn) arrives in private detective Cormoran Strike’s (Tom Burke) office telling the story of a child he saw strangled many years ago. Strike is simultaneously hired by government minister Jasper Chiswell (Robert Glenister) to investigate Billy’s brother, Jimmy Knight (Nick Blood), who is blackmailing him. As Strike and his partner Robin work to determine how the cases might be connected, Robin goes undercover in the House of Commons.
　　However, Robin’s private life is unravelling as her relationship with Matthew (Kerr Logan) feels the full strain of her commitment to Strike and her work. At the same time, Strike has his own issues, a girlfriend who confesses she loves him and his complicated ex Charlotte (Natasha O’Keeffe) who is pregnant and back in the frame.