While performing a diagnostic on the holodeck, Lt. Barclay accidentally releases Professor Moriarty, who has been aware of the passage of time since Picard stored his program four years ago. Professor Moriarty escapes from the realm of computer simulation into reality, where he takes control of the ship.

Daniel Davis (Professor James Moriarty)
Dwight Schultz (Lt. Reginald Barclay)
Stephanie Beacham (Countess Regina Bartholomew)
Clement von Franckenstein (Gentleman)
Majel Barrett (Computer Voice)

• Moriarty trapped Picard, Data and Barclay in a holodeck simulation within the holodeck. Once Data discovered it, they used the same trick on him. They placed Moriarty and the Countess' programs into a memory cube, which will provide enough data to last them a lifetime.
  
• Professor Moriarty was stored in the second season episode "Elementary, Dear Data".
  
• Barclay was introduced in Season 3's "Hollow Pursuits".
  
• Even though Sir Arthur Conan Doyle's estate earlier refused to allow any more "Sherlock Holmes" episodes, they decided to allow TNG to license the characters for this episode (for a fee, of course).
  
• Troi continues to wear her standard Starfleet uniform after Jellico's departure, after he ordered her to in "Chain of Command, Part I".