What is the difference between dedication and obsession?

While investigating the murder of two known offenders with connections to a notorious, recently escaped London drug trafficker, the Scotland Yard forensic team headed by DCI Eric Shaw becomes involved in a child abuse case. A nurse had accused a mother of a series of violent, feverish attacks on her ten-year-old son, Jimmy. The woman would exasperate the condition of her child to draw the attention and compassion of health workers at the hospital.
Eric learned of this accusation by chance because he is dating Catherine Foulger, the paediatrician looking after the child. She is an old flame, and he is seeing her again in the hope of putting some order back in his life after discovering the identity of the serial killer nicknamed Black Death.
But this is a relationship his former partner Adele Pennington, still working as an investigator in Forensic Services, has not really accepted.

DCI Eric Shaw returns.