D.I. Mark Cascoyne was a police officer at Dee Valley Police Station, and the superior of Calvin Valentine. In October 2008, he investigated the fake death of Jack Osborne. Later in the month, he comforted Calvin's wife Carmel, who was contemplating quitting her job after co-worker Niall Rafferty tried to kill the McQueen family in a church explosion. After getting shoplifting charges against Carmel's sister Jacqui McQueen dropped, she bought him a drink and ended up sleeping with him. The pair were caught by Jacqui's boyfriend Tony Hutchinson and Dom Reilly threw him out of the flat.
Carmel comforted Mark when his partner left him over his liaison with Jacqui, leaving Calvin jealous. Mark began texting Calvin from Calvin's phone and tried to kiss Carmel - witnessed by Calvin - but she rejected him. In November 2008, Mark led a raid for stolen goods at The Loft, but nothing was found as Calvin had warned owner Warren Fox. In revenge for his actions towards Calvin and his club, Warren followed Mark into the woods and beat him unconscious with a plank of wood. Mark - in retaliation - had Calvin beat Warren with a baseball bat.
In December 2008, Mark and Calvin arrested Tony Hutchinson for sleeping with 15-year-old Theresa McQueen. In May 2009, he became aware of Calvin's deals with Warren. Calvin told him everything and that he had plans to get Warren to confess to Sean Kennedy murder. Calvin recorded a confession of both Sean and Louise's murders. However, the recorder had broken and the evidence was lost. Calvin was then sacked. In June, Mark made enquiries about Warren's murder and searched for suspect Justin Burton. He then arrested Clare Devine.