DS Sarah Collins (Gemma Whelan) and DC Steve Bradshaw (Jimmy Akingbola) are called to an incident at Portland Tower in south-east London. Two people have fallen to their deaths from the tower: long serving police officer, PC Hadley Matthews (Nick Holder), and 15-year-old Libyan refugee, Farah Mehenni (Lola Elsokari). On the roof Sarah finds a traumatised five-year-old boy and a rookie police officer, PC Lizzie Adama (Tahirah Sharif) - the only witnesses to what happened. Before Sarah and Steve can interview Lizzie she disappears. The more they investigate, the more complex the case becomes and they are both forced to confront challenging questions about what it really means to be a police officer.