Three probation period rookie police officers in the Police Service of Northern Ireland are assigned to Blackthorn Station in Belfast. One of the new officers, Grace, is a mum in her 40s, who left her job as a social worker to join the Police Service, but only weeks into the job she wonders if she's made the right decision. Her fellow rookies are Annie and Tommy, who themselves are struggling under the immense pressure of the job. Facing criminal gangs and divided communities, they don't know who to trust. The odds are, they won't all make it.