At the very beginning, Dr. Spencer Reid sketches the portrait of Nathan Harris (Anton Yelchin) in an attempt to identify him as a potential serial killer. This sketch was actually made by Matthew Gray Gubler in real life.
Nathan mentions that he attended Dr. Reid's lecture on Mill Creek Killer. The case in question occurred in The Last Word (2006).
Anton Yelchin guest stars as Nathan Harris. He previously starred in Huff (2004) with Paget Brewster (Agent Emily Prentiss) and played her son.
During the profiling meeting, a whiteboard behind Gideon lists the "B. A. U. 3R's": "Restrictions," "Registrations," and "Regulations."
At the beginning of the episode, Nathan Harris references hearing about Reid's work on the Mill Creek Killer as seen in The Last Word. In that episode, the other killer was called the Hallow Man. At the end of this episode, the poem Reid recited is from T.S. Eliot's The Hallow Men.