Patrick Dempsey joins the cast of "Dexter: Original Sin" as Aaron Spencer, Captain of Miami Metro Homicide.

Grey's Anatomy alum Patrick Dempsey joins the cast of Dexter: Original Sin, a prequel series to the popular Showtime show Dexter. Set in 1991, the series follows a young Dexter Morgan as he transitions from student to avenging serial killer, guided by his father Harry. Dempsey will play Aaron Spencer, the Captain of the Miami Metro Homicide Department, with a longstanding relationship with Harry Morgan, played by Christian Slater.