Schizophrenic Michael Simms threatens Shelley Cooper with a knife and when the woman is later killed, Manson orders CO19 to shoot him, assuming Simms is the culprit. In the subsequent inquiry by the department of professional standards, who also subject other officers in CID to criticism, the DI's judgement is called into question by his own superior officer, and he faces the prospect that his career could be over. (thetvdb.com)