6x13 - Diagnosis: Murder

Newly retired detective Harry Trumble returns to the force when it appears 'The Clown Killer', a murderer Trumble chased for more than a decade, has emerged from obscurity to kill again. Mark is reluctant to help Harry investigate the Clown, in part due to a rift between the two, but also because Mark suspects Harry has something to do with the Clown's reappearance.