Sam Ryan assists the police in the investigation of a multi-vehicle collision that results in the death of 11 people. She is joined by two new colleagues: Dr. Leo Dalton, an experienced pathologist from Sheffield and Dr. Harry Cunningham. As they try to determine the causes of the accident, they find a severed arm that does not belong to any of the victims. They also have to deal with DCI Carol Deacon whose role in the investigation is not quite clear to them. Sam and Leo have somewhat different approaches to their job, particularly in how they deal with the police. Harry performs his first autopsy on a child.