Ridley follows charismatic Detective Inspector Alex Ridley, who is retiring from the police after years of dedicated service. After twenty-five years as a homicide copper, Ridley has been nudged out of the job he loves. But the prospect of retirement is not a particularly welcome one as he's been cut off in his prime. Ridley's replacement is Acting DI Carol Farman, his former protégée whom Ridley mentored for many years. When he's enlisted by Carol as a police consultant on a complex and compelling murder case, the investigation takes a dark and unexpected turn. Turning to her old mentor for support, Carol is keen to access Ridley's unique insight into crime-solving which has served them so well in the past.