Finally an episode that is everything Masterpiece (#4.8) aspired to be, yet failed in its we-didn't-really-think-this-through cleverness. In a series that is mainly episodic, this one also contains several nods to long-time viewers and establishes what probably is as close to mythology (in TV trope sense, not cultural sense) as Criminal Minds will ever get. Kudos! Finally we have an episode that measures up to the standards set by "L.D.S.K." (#1.6), "The Fisher King: Part 1" (#1.22), "Revelations" (#2.15), "3rd Life" (#3.12), and "Lo-Fi" (#3.20).
With an episode like this, it's excellence cannot be thoroughly analyzed without spoilers. Since this seems to be universally a highly regarded episode, there is no need to explain in detail why it is good. Suffice to say, the script is amazing, surprising both plot and character-wise: Hotch is finally made interesting! The BAU is up against a true Chessmaster, not a wannabe like the clown in Masterpiece. Brutal, tense and inventive, this is a must-see. The only reason it's "only" a 9/10? It would be a spoiler...